Éamon de Valera was born on 14 October 1882 in New York City, the son of Catherine Coll—an Irishwoman originally from Bruree, County Limerick—and Juan Vivion de Valera, described on the birth certificate as a Spanish Artist born in the Basque Country. His parents were reportedly married on 18 September 1881 at St Patrick's Church in Jersey City, New Jersey, but archivists have not located any marriage certificate or any birth, baptismal, or death certificate information for anyone called Juan Vivion de Valera (nor for "de Valeros", an alternative spelling). On de Valera's original birth certificate, his name is given as George de Valero and his father is listed as Vivion de Valero. Although he was known as Edward de Valera before 1901, a fresh birth certificate was issued in 1910, in which his first name was officially changed to Edward and his father's surname given as "de Valera". As a child, he was known as "Eddie" or "Eddy".

According to Coll, Juan Vivion died in 1885 leaving Coll and her child in poor circumstances. Éamon was taken to Ireland by his uncle Ned at the age of two. Even when his mother married a new husband in the mid-1880s, he was not brought back to live with her, but was reared instead by his grandmother, Elizabeth Coll, her son Patrick and her daughter Hannie, in County Limerick. He was educated locally at Bruree National School, County Limerick and C.B.S. Charleville, County Cork. Aged sixteen, he won a scholarship. He was not successful in enrolling at two colleges in Limerick, but was accepted at Blackrock College, Dublin, at the instigation of his local curate.

Always a diligent student, at the end of his first year in Blackrock College he was Student of the Year. He also won further scholarships and exhibitions and in 1903 was appointed Teacher of mathematics at Rockwell College, County Tipperary. It was here that de Valera was first given the nickname "Dev" by a teaching colleague, Tom O'Donnell. In 1904, he graduated in mathematics from the Royal University of Ireland. He then studied for a year at Trinity College Dublin but, owing to the necessity of earning a living, did not proceed further and returned to teaching, this time at Belvedere College. In 1906, he secured a post as Teacher of mathematics at Carysfort Teachers' Training College for women in Blackrock, County Dublin. His applications for professorships in colleges of the National University of Ireland were unsuccessful, but he obtained a part-time appointment at Maynooth and also taught mathematics at various Dublin schools, including Castleknock College (1910–1911; under the name Edward de Valera) and Belvedere College.

De Valera was among the few republican Leaders the British did not execute. It has been argued that his life was saved by four facts. First, he was one of the last to surrender and he was held in a different prison from other Leaders, thus his execution was delayed by practicalities. Second, the US Consulate in Dublin made representations before his trial (i.e., was he actually a United States citizen and if so, how would the United States react to the execution of one of its citizens?) while the full legal situation was clarified. The UK was trying to bring the US into the war in Europe at the time, and the Irish American vote was important in US politics, though this did not prevent the execution of Tom Clarke who had been a naturalised American citizen since 1905. Third, when Lt-Gen Sir John Maxwell reviewed his case he said, "Who is he? I haven't heard of him before. I wonder would he be likely to make trouble in the Future?" On being told that de Valera was unimportant, he commuted the court-martial's death sentence to life imprisonment. De Valera had no Fenian family or personal background and his MI5 file in 1916 was very slim, detailing only his open membership in the Irish Volunteers. Fourth, by the time de Valera was court-martialled on 8 May, political pressure was being brought to bear on Maxwell to halt the executions; Maxwell had already told the Prime Minister H. H. Asquith that only two more were to be executed, Seán Mac Diarmada and James Connolly, although they were court-martialled the day after de Valera. His late trial, representations made by the American Consulate, his lack of Fenian background and political pressure all combined to save his life, though had he been tried a week earlier he would probably have been shot.

There were occasions when de Valera seriously contemplated the religious life like his half-brother, Fr Thomas Wheelwright, but ultimately he did not pursue this vocation. As late as 1906, when he was 24 years old, he approached the President of Clonliffe Seminary in Dublin for advice on his vocation. De Valera was throughout his life portrayed as a deeply religious man, who in death asked to be buried in a religious habit. His biographer, Tim Pat Coogan, speculated that questions surrounding de Valera's legitimacy may have been a deciding factor in his not entering religious life. Being illegitimate would have been a bar to receiving priestly orders, but not to becoming a lay member of a religious order.

De Valera fathered five sons: Vivion (1910–82), Éamon (1913–86), Brian (1915–36), Rúaidhrí (1916–78), and Terence (Terry; 1922–2007); and two daughters: Máirín (1912–84) and Emer (1918–2012). Brian de Valera predeceased his parents.

While he was already involved in the Gaelic Revival, de Valera's involvement in the political revolution began on 25 November 1913 when he joined the Irish Volunteers formed to oppose the Ulster Volunteers and ensure the enactment of the Irish Parliamentary Party's Third Home Rule Act won by its leader John Redmond. After the outbreak of World War I in August 1914, de Valera rose through the ranks and it was not long before he was elected captain of the Donnybrook company. Preparations were pushed ahead for an armed revolt, and he was made commandant of the Third Battalion and adjutant of the Dublin Brigade. He took part in the Howth gun-running. He was sworn by Thomas MacDonagh into the oath-bound Irish Republican Brotherhood, which secretly controlled the central executive of the Volunteers. He opposed secret societies, but this was the only way he could be guaranteed full information on plans for the Rising.

De Valera's supporters and detractors argue about his bravery during the Easter Rising. His supporters claim he showed leadership skills and a capacity for meticulous planning. His detractors claim he suffered a nervous breakdown during the Rising. According to accounts from 1916, de Valera was seen running about, giving conflicting orders, refusing to sleep and on one occasion, having forgotten the password, almost getting himself shot in the dark by his own men. According to one account, de Valera, on being forced to sleep by one subordinate who promised to sit beside him and wake him if he was needed, suddenly woke up, his eyes "wild", screaming, "Set fire to the railway! Set fire to the railway!" Later in the Ballykinlar internment Camp, one de Valera loyalist approached another internee, a medical Doctor, recounted the story, and asked for a medical opinion as to de Valera's condition. He also threatened to sue the Doctor, Future Fine Gael Teachta Dála (TD) and minister, Dr. Tom O'Higgins, if he ever repeated the story. The British reportedly, however, considered de Valera's forces the best-trained and best-led among the rebels. De Valera's latest biographer, Anthony J. Jordan, writes of this controversy, "Whatever happened in Boland's Mills, or any other garrison, does not negate or undermine in any way the extraordinary heroism of "Dev" and his comrades".

After imprisonment in Dartmoor, Maidstone and Lewes prisons, de Valera and his comrades were released under an amnesty in June 1917. On 10 July 1917 he was elected member of the House of Commons for East Clare (the constituency which he represented until 1959) in a by-election caused by the death of the previous incumbent Willie Redmond, brother of the Irish Party Leader John Redmond who had died fighting in World War I. In the 1918 general election he was elected both for that seat and Mayo East. Because most other Irish rebellion Leaders were dead, in 1917 he was elected President of Sinn Féin, the party which had been blamed incorrectly for provoking the Easter Rising. This party became the political vehicle through which the survivors of the Easter Rising channeled their republican ethos and objectives. The previous President of Sinn Féin, Arthur Griffith, had championed an Anglo-Irish dual-monarchy based on the Austro-Hungarian model, with independent legislatures for both Ireland and Britain.

Meanwhile in Ireland, conflict between the British authorities and the Dáil (which the British declared illegal in September 1919) escalated into the Irish War of Independence. De Valera left day-to-day government, during his eighteen-month absence in the United States, to Michael Collins, his 29-year-old Minister for Finance. De Valera and Collins would later become opponents during the Irish Civil War.

In recent decades, his role in Irish history has no longer been unequivocally seen by historians as a positive one, and a biography by Tim Pat Coogan alleges that his failures outweigh his achievements, with de Valera's reputation declining while that of his great rival in the 1920s, Michael Collins, is rising. The most recent work on de Valera by Historian Diarmaid Ferriter presents a more positive picture of de Valera's legacy. Bertie Ahern, at a book launch for Diarmaid Ferriter's biography of de Valera, described de Valera's achievements in political leadership during the formative years of the state:

Ireland's dominant political personality for many decades, de Valera received numerous honours. He was elected Chancellor of the National University of Ireland in 1921, holding the post until his death. Pope John XXIII bestowed on him the Order of Christ. He received honorary degrees from universities in Ireland and abroad. In 1968 he was elected a Fellow of the Royal Society (FRS), a recognition of his lifelong interest in mathematics. He also served as a member of the Parliament of Northern Ireland (for Down from 1921 to 1929 and for South Down from 1933 to 1937), although he held to the Republican policy of abstentionism and did not take his seat in Stormont. He retired from the Presidency in June 1973, having served for fourteen years.

Though nominally head of the anti-Treatyites, de Valera had little influence. He does not seem to have been involved in any fighting and had little or no influence with the military republican leadership - headed by IRA Chief of Staff, Liam Lynch. De Valera and the anti-Treaty TDs formed a "republican government" on 25 October 1922 from anti-Treaty TDs to "be temporarily the Supreme Executive of the Republic and the State, until such time as the elected Parliament of the Republic can freely assemble, or the people being rid of external aggression are at liberty to decide freely how they are to be governed". However it had no real authority and was a pale Shadow of the republican Dáil government of 1919–21, which had provided an alternative government to the British administration.

On 30 May 1923, the IRA's new Chief of Staff Frank Aiken (Lynch had been killed) called a ceasefire and ordered volunteers to "dump arms". De Valera, who had wanted an end to the internecine fighting for some time, backed the ceasefire order with a message in which he called the anti-Treaty fighters "the Legion of the Rearguard", saying that "The Republic can no longer be successfully defended by your arms. Further sacrifice on your part would now be in vain and the continuance of the struggle in arms unwise in the national interest and prejudicial to the Future of our cause. Military victory must be allowed to rest for the moment with those who have destroyed the Republic. Other means must be sought to safeguard the nation's right."

After the IRA dumped their arms rather than surrender them or continue a now fruitless war, de Valera returned to political methods. In 1924 he was arrested in Newry for "illegally entering Northern Ireland" and held in solitary confinement for a month in Crumlin Road Gaol, Belfast.

However, one of his colleagues, Seán Lemass, convinced de Valera to found a new republican party. In March 1926, with Lemass, Constance Markievicz and others, de Valera formed a new party, Fianna Fáil (The Warriors of Destiny), a party that was to dominate 20th-century Irish politics. While Sinn Féin still held to an abstentionist line, Fianna Fáil was dedicated to republicanising the Free State from within if it gained power.

The oath was largely the work of Michael Collins and based on three sources: British oaths in the dominions, the oath of the Irish Republican Brotherhood and a draft oath prepared by de Valera in his proposed Treaty alternative, "Document No. 2"). De Valera began a legal case to challenge the requirement that members of his party take the Oath, but the assassination of the Vice-President of the Executive Council (deputy prime minister) Kevin O'Higgins led the Executive Council under W. T. Cosgrave to introduce a Bill requiring all Dáil candidates to promise on oath that if they were elected they would take the Oath of Allegiance. Forced into a corner, and faced with the option of staying outside politics forever or taking the oath and entering, de Valera and his TDs took the Oath of Allegiance in 1927, though de Valera himself described the Oath as "an empty political formula."

During the 1930s, de Valera had systematically stripped down the Irish Free State constitution that had been drafted by a committee under the nominal chairmanship of his rival, Michael Collins. In reality, de Valera had been able to do that only due to three reasons. First, though the 1922 constitution originally required a public plebiscite for any amendment beyond eight years after its passage, the Free State government under W. T. Cosgrave had amended that period to sixteen years. This meant that, until 1938, the Free State constitution could be amended by the simple passage of a Constitutional Amendment Act through the Oireachtas. Secondly, while the Governor-General of the Irish Free State could reserve or deny Royal Assent to any legislation, from 1927, the power to advise the governor-general to do so no longer rested with the British government in London but with His Majesty's Government in the Irish Free State, which meant that, in practice, the Royal Assent was automatically granted to legislation; the government was hardly likely to advise the governor-general to block the enactment of one of its own bills. Thirdly, in theory the constitution had to be in keeping with the provisions of the Anglo-Irish Treaty, the fundamental law of the state. However, that requirement had been removed only a short time before de Valera gained power.

The Oath of Allegiance was abolished, as were appeals to the Judicial Committee of the Privy Council. The opposition-controlled Senate, when it protested and slowed down these measures, were also abolished. In 1931, the British parliament had passed the Statute of Westminster, which established the legislative equal status of the self-governing Dominions of the then British Commonwealth, including the Irish Free State, to one another and the United Kingdom. Though a few constitutional links between the Dominions and the United Kingdom remained, this is often seen as the moment at which the Dominions became fully sovereign states.

De Valera took charge of Ireland's foreign policy as well by also acting as Minister for External Affairs. In that capacity, he attended meetings of the League of Nations. He was President of the Council of the League on his first appearance at Geneva in 1932 and, in a speech that made a worldwide impression, appealed for genuine adherence by its members to the principles of the covenant of the league. In 1934, he supported the admission of the Soviet Union into the league. In September 1938, he was elected nineteenth President of the Assembly of the League, a tribute to the international recognition he had won by his independent stance on world questions.

The ACA changed its name to the National Guard under O'Duffy and adopted the uniform of black berets and blue shirts, using the straight armed salute, and were nicknamed The Blueshirts. They were outwardly fascist and planned a march in August 1933 through Dublin to commemorate Michael Collins, Kevin O'Higgins, and Arthur Griffith. This march struck parallels with Mussolini's march on Rome (1922), in which he had created the image of having toppled the democratic government in Rome. De Valera revived a military tribunal, which had been set up by the previous administration, to deal with the matter. O'Duffy backed down when the National Guard was declared an illegal organisation and the march was banned. Within a few weeks, O'Duffy's followers merged with Cumann na nGaedhael and the Centre Party to form United Ireland, or Fine Gael, and O'Duffy became its President. Smaller local marches were scheduled for the following weeks, under different names. Internal dissension set in when the party's TDs distanced themselves from O'Duffy's extreme views, and his movement fell asunder.

De Valera, in his capacity as prime minister of His Majesty's Government in the Irish Free State, wrote in July 1936 to King Edward VIII in London indicating that he planned to introduce a new constitution, the central part of which was to be the creation of an office de Valera provisionally intended to call President of Saorstát Éireann (Uachtarán Shaorstát Éireann), which would replace the governorship-general. De Valera used the sudden abdication of Edward VIII as king to pass two bills: one amended the constitution to remove all mention of the monarch and governor-general, while the second brought the monarch back, this time through statute law, for use in representing the Irish Free State at diplomatic level. With the implementation of the new constitution, named in Irish as Bunreacht na hÉireann (meaning the Constitution of Ireland), the title ultimately given to the President was President of Ireland (Uachtarán na hÉireann).

A notable failure was his attempt to reverse the provision of the 1937 Constitution in relation to the electoral system. On retiring as Taoiseach in 1959, he proposed that the Proportional Representation system enshrined in that constitution should be replaced. De Valera argued that Proportional Representation had been responsible for the instability that had characterised much of the post war period. A constitutional referendum to ratify this was defeated by the people. One aspect of de Valera's legacy is that since the foundation of the state, a de Valera has nearly always served in Dáil Éireann. Éamon de Valera served until 1959, his son, Vivion de Valera, was also a Teachta Dála (TD). Éamon Ó Cuív, his grandson, is currently a member of the Dáil while his granddaughter, Síle de Valera is a former TD. Both have served in ministries in the Irish Government.

Persistent claims that De Valera sent a personal note of congratulation to Subhas Chandra Bose upon his declaration of the Azad Hind (Free India) government in 1943 have been shown to be inaccurate, and largely a misrepresentation by Japanese consular staff in Dublin of a statement by a small and unofficial Republican group unconnected to the Irish government.

Controversially, de Valera formally offered his condolences to the German Minister in Dublin on the death of Adolf Hitler in 1945, in accordance with diplomatic protocol. This did some damage to Ireland, particularly in the United States – and soon afterwards de Valera had a bitter exchange of words with Winston Churchill in two famous radio addresses after the end of the war in Europe.

By September 1939, a general European war was imminent. On 2 September, de Valera advised Dáil Éireann that neutrality was the best policy for the country. This policy had overwhelming political and popular support, though some advocated Irish participation in the War on the Allied side, while others, seeing "England's difficulty as Ireland's Opportunity", were pro-German. Strong objections to conscription in the North were voiced by de Valera. In June 1940, to encourage the neutral Irish state to join with the Allies, Winston Churchill indicated to de Valera that the United Kingdom would push for Irish unity, but believing that Churchill could not deliver, de Valera declined the offer. The British did not inform the Government of Northern Ireland that they had made the offer to the Dublin government, and De Valera's rejection was not publicised until 1970. The government secured wide powers for the duration of the Emergency, such as internment, censorship of the press and correspondence, and the government control of the economy. The Emergency Powers Act lapsed on 2 September 1946, though the State of Emergency declared under the constitution was not lifted until the 1970s. This status remained throughout the war, despite pressure from Chamberlain and Churchill. However, de Valera did respond to a request from Northern Ireland for fire tenders to assist in fighting fires following the Belfast Blitz.

In social policy, de Valera's first term as Taoiseach saw the introduction (in 1947) of means-tested allowances for people suffering from infectious diseases.

De Valera, now leader of the opposition, left the actual parliamentary practice of opposing the government to his deputy, Seán Lemass, and himself embarked on a world campaign to address the issue of partition. He visited the United States, Australia, New Zealand and India, and in the latter country, was the last guest of the Governor-General, Lord Mountbatten of Burma, before he was succeeded by the first Indian-born Governor-General. In Melbourne, Australia, de Valera was feted by the powerful Catholic Archbishop Daniel Mannix, at the centenary celebrations of the diocese of Melbourne. He attended mass-meetings at Xavier College, and addressed the assembled Melbourne Celtic Club. In October 1950, just thirty years after his dramatic escape from Lincoln Gaol, he returned to Lincoln and received the freedom of the gaol. The Anti-Partition of Ireland League of Great Britain marked the occasion with a dinner in his honour and the toast was 'Anglo-Irish Friendship'. A key message in de Valera's campaign was that Ireland could not join the recently established North Atlantic Treaty Organization as long as Northern Ireland was in British hands; although the coalition government favored alliance with NATO, de Valera's approach won more widespread support and prevented the state from signing onto the treaty.

Returning to Ireland, during the Mother and Child Scheme crisis that racked the First Inter-Party Government, de Valera kept a dignified silence as Leader of the Opposition, preferring to stay aloof from the controversy. That stance helped return de Valera to power in the 1951 general election, but without an overall majority. His and Fianna Fáil's popularity was short-lived, however; his government introduced severe, deflationary budgetary and economic policies in 1952, causing a political backlash that cost Fianna Fáil several seats in the Dáil in by-elections of 1953 and early 1954. Faced with a likely loss of confidence in the Dáil, de Valera instead called an election in May 1954, in which Fianna Fáil was defeated and a Second Inter-Party Government was formed with Costello again as Taoiseach.

De Valera's final term as Taoiseach also saw the passage of numerous reforms in health and welfare. In 1952, unemployment insurance was extended to male agricultural employees, child allowances were extended to the second child, and a maternity allowance for insured women was introduced. A year later, eligibility for maternity and child services and public hospital services was extended to circa 85% of the population.

On 16 September 1953 de Valera met Churchill for the first and only time, at 10 Downing Street. (The two men had seen each other at a party in 1949, but without speaking.) He surprised the UK Prime Minister by claiming that if he had been in office in 1948 Ireland would not have left the Commonwealth.

Like the first coalition government, the second lasted only three years. At the general election of 1957, de Valera, then in his seventy-fifth year, won an absolute majority of nine seats, the greatest number he had ever secured. This was the beginning of another sixteen-year period in office for Fianna Fáil. A new economic policy emerged with the First Programme for Economic Expansion. In July 1957, in response to the Border Campaign (IRA), Part II of the Offences Against the State Act was re-activated and he ordered the internment without trial of Republican suspects, an action which did much to end the IRA's campaign.

De Valera was inaugurated President on 25 June 1959. He was re-elected President in 1966 aged 84, until 2013 a world record for the oldest elected head of state. At his retirement in 1973 at the age of 90, he was the oldest head of state in the world.

Assessments of de Valera's career have varied; he has often been characterised as a stern, unbending, devious, and divisive Irish Politician. Biographer Tim Pat Coogan sees his time in power as being characterised by economic and cultural stagnation, while Diarmaid Ferriter argues that the stereotype of de Valera as an austere, cold and even backward figure was largely manufactured in the 1960s and is misguided.

As President, de Valera received many state visits, including the 1963 visit of American President John F. Kennedy. Five months later de Valera attended the state funeral for Kennedy in Washington, D.C. and accompanied a group of 24 Defence Forces cadets who performed a silent drill at his grave site. In June 1964 he returned to Washington as the second President of Ireland to address the United States Congress.

In 1966 the Dublin Jewish community arranged the planting and dedication of the Éamon de Valera Forest in Israel, near Nazareth, in recognition of his support for Ireland's Jews.

In 1969, seventy-three countries sent goodwill messages to NASA for the historic first lunar landing. These messages still rest on the lunar surface and de Valera's message on behalf of Ireland stated, "May God grant that the skill and courage which have enabled man to alight upon the Moon will enable him, also, to secure peace and happiness upon the Earth and avoid the danger of self-destruction."

The specific recognition of Roman Catholicism was deleted by the Fifth Amendment of the Constitution of Ireland (1973) and the prohibition of divorce was removed by the Fifteenth Amendment of the Constitution of Ireland (1996). Nevertheless, the Irish Supreme Court declared in 1973 that the 1935 contraception legislation was not repugnant to the Constitution and therefore remained valid.

Éamon de Valera died from pneumonia and heart failure in Linden Convalescent Home, Blackrock, County Dublin, on 29 August 1975 aged 92. His wife, Sinéad de Valera, four years his senior, had died the previous January, on the eve of their 65th wedding anniversary. His body lay in state at Dublin Castle and was given a full state funeral on 3 September at St Mary's Pro-Cathedral, which was broadcast on national television. He is buried in Dublin's Glasnevin Cemetery with his wife and children.

De Valera never organised Fianna Fáil in Northern Ireland and it was not until 7 December 2007 that Fianna Fáil was registered there by the UK Electoral Commission.

After de Valera had spent sixteen years in power—without answering the crucial questions of partition and republican status—the public demanded a change from Fianna Fáil government. In the 1948 election, de Valera lost the outright majority he'd enjoyed since 1933. It initially looked like the National Labour Party would give Fianna Fáil enough support to stay in office as a minority government, but National Labour insisted on a formal coalition agreement—something de Valera was unwilling to concede. However, while Fianna Fáil was six seats short of a majority, it was still by far the largest party in the Dáil, with 37 more TDs than the next largest party, Fine Gael (the successor to Cumann na nGaedheal). Conventional wisdom held that de Valera would remain Taoiseach with the support of independents.

In January 2017 his grave was vandalised. A man was arrested and charged with Criminal damage, the case being adjourned until June.