Juan Carlos was born to Infante Juan, Count of Barcelona and Princess María Mercedes of Bourbon-Two Sicilies in Rome, Italy, where his grandfather, King Alfonso XIII of Spain, and other members of the Spanish Royal Family lived in exile following the proclamation of the Second Spanish Republic in 1931. He was baptized as Juan Carlos Alfonso Víctor María de Borbón y Borbón-Dos Sicilias. He was given the name Juan Carlos after his father and maternal grandfather, Prince Carlos of Bourbon-Two Sicilies.

Juan Carlos has two sisters: Infanta Pilar, Duchess of Badajoz (born 1936); and Infanta Margarita, Duchess of Soria (born 1939). He also had a younger brother, Alfonso.

Juan Carlos I (Spanish: [xwaŋˈkaɾlos]; Juan Carlos Alfonso Víctor María de Borbón y Borbón-Dos Sicilias, born 5 January 1938) reigned as King of Spain from 1975 until his abdication in 2014.

His early life was dictated largely by the political concerns of his father and General Franco. He moved to Spain in 1948 to be educated there after his father persuaded Franco to allow it. He began his studies in San Sebastián and finished them in 1954 at the San Isidro Institute in Madrid. He then joined the army, doing his officer training from 1955 to 1957 at the Military Academy of Zaragoza.

Juan Carlos is also the alleged illegitimate father of Alberto Sola, born in Barcelona in 1956, and of a Belgian woman, Ingrid Sartiau, who has filed a paternity suit; complete sovereign immunity had prevented the suit prior to his abdication.

In 1957, Juan Carlos spent a year in the naval school at Marín, Pontevedra, and another in the Air Force school in San Javier in Murcia. In 1960–61, he studied Law, International Political Economy and Public Finance at Complutense University. He then went to live in the Palace of Zarzuela and began carrying out official engagements.

The dictatorial regime of Francisco Franco came to power during the Spanish Civil War, which pitted a government of democrats, anarchists, socialists, and communists, supported by the Soviet Union and international volunteers, against a rebellion of conservatives, monarchists, nationalists, and fascists, supported by both Hitler and Mussolini, with the rebels ultimately winning. Franco's authoritarian government remained dominant in Spain until the 1960s. With Franco's increasing age, left-wing protests increased, while at the same time, the far right factions demanded the return of a hardline absolute monarchy. At the time, the heir to the throne of Spain was Juan de Borbón (Count of Barcelona), the son of the late Alfonso XIII. However, General Franco viewed him with extreme suspicion, believing him to be a liberal who was opposed to his regime.

Juan Carlos was married in Athens at the Church of Saint Dennis on 14 May 1962, to Princess Sophia of Greece and Denmark, daughter of King Paul of Greece. She converted from her Greek Orthodox religion to Roman Catholicism. Also in 1962, a Roman Catholic wedding was performed in the Pauline Chapel the Basilica of Santa Maria Maggiore in Rome.

Juan Carlos met and consulted Franco many times while heir apparent and often took part in official and ceremonial state functions, standing alongside the dictator, much to the anger of hardline republicans and more moderate liberals, who hoped that Franco's death would bring in an era of reform. During 1969–1975, Juan Carlos publicly supported Franco's regime. Although Franco's health worsened during those years, whenever he did appear in public, from state dinners to military parades, it was in Juan Carlos's company. Juan Carlos continued to praise Franco and his government for the economic growth and positive changes in Spain. However, as the years progressed, Juan Carlos began meeting secretly with political opposition Leaders and exiles, who were fighting to bring liberal reform to the country. He also had secret conversations with his father over the telephone. Franco, for his part, remained largely oblivious to the prince's actions and denied allegations from his ministers and advisors that Juan Carlos was in any way disloyal to his vision of the regime.

In 1972, Juan Carlos, a keen Sailor, competed in the Dragon class event at the Olympic Games, finishing 15th. In their summer holidays, the whole family meets in Marivent Palace (Palma de Mallorca) and the Fortuna yacht, where they take part in sailing competitions. The king has manned the Bribón series of yachts. In winter, the family often go skiing in Baqueira-Beret and Candanchú (Pyrenees). At present, his hobbies include classic sailing boats.

During periods of Franco's temporary incapacity in 1974 and 1975, Juan Carlos was acting head of state. On 30 October 1975, Franco gave full control to Juan Carlos; he died three weeks later, on 20 November. On 22 November, two days after Franco's death, the Cortes Generales proclaimed Juan Carlos King of Spain. In his address to the Cortes, Juan Carlos spoke of three factors: historical tradition, national laws, and the will of the people, and in so doing referred to a process dating back to the Civil War of 1936–39. On 27 November, a Mass of the Holy Spirit was celebrated in the church of San Jerónimo el Real in Madrid to inaugurate his reign. He opted not to call himself Juan III or Carlos V, but Juan Carlos I. Juan Carlos is reported to have been pressured by Valéry Giscard d'Estaing to personally tell Chilean dictator Augusto Pinochet, who had traveled to Spain for Franco's funeral, not to attend his coronation.

Juan Carlos's accession met with relatively little parliamentary opposition. Some members of the Movimiento Nacional voted against recognising him, and more against the 1976 Law for Political Reform. But even most Movimiento members supported both measures. Juan Carlos quickly instituted reforms, to the great displeasure of Falangist and conservative (monarchist) elements, especially in the military, who had expected him to maintain the authoritarian state. In July 1976, Juan Carlos dismissed prime minister Carlos Arias Navarro, who had been attempting to continue Francoist policies in the face of the king's attempts at democratisation. He instead appointed Adolfo Suárez, a former leader of the Movimiento Nacional, as prime minister, who would go on to win the following year's election and become the first democratically elected leader of the new regime.

On 15 June 1977, Spain held its first post-Franco democratic elections. In 1978, the government promulgated a new constitution that acknowledged Juan Carlos as rightful heir of the Spanish dynasty and king; specifically, Title II, Section 57 asserted Juan Carlos's right to the throne of Spain by dynastic succession in the Bourbon tradition, as "the legitimate heir of the historic dynasty" rather than as the designated successor of Franco. The Constitution was passed by the democratically elected Constituent Cortes, ratified by the people in a referendum (6 December) and then signed into law by the King before a solemn meeting of the Cortes.

Expected to continue Franco's legacy, Juan Carlos, however, soon after his accession introduced reforms to dismantle the Francoist regime and begin the Spanish transition to democracy. This led to the approval of the Spanish Constitution of 1978 in a referendum, which re-established a constitutional monarchy. In 1981, Juan Carlos played a major role in preventing a coup that attempted to revert Spain to Francoist government in the King's name. In 2008, he was considered the most popular leader in all Ibero-America. Hailed for his role in Spain's transition to democracy, the King and the monarchy's reputation began to suffer after controversies surrounding his family arose, exacerbated by an elephant-hunting trip he undertook during a time of financial crisis in Spain. In 2014, Juan Carlos, citing personal reasons, abdicated in favour of his son, who acceded to the throne as Felipe VI.

When Juan Carlos became king, Communist leader Santiago Carrillo nicknamed him Juan Carlos the Brief, predicting that the monarchy would soon be swept away with the other remnants of the Franco era. After the collapse of the attempted coup, however, in an emotional statement, Carrillo remarked: "Today, we are all monarchists." Public support for the monarchy among democrats and leftists before 1981 had been limited; following the king's handling of the coup it increased significantly.

On paper, Juan Carlos retained fairly extensive reserve powers. He was the guardian of the constitution, and was responsible for ensuring that it was obeyed. In practice, since the passage of the Constitution (and especially since 1982) he took a mostly non-partisan and representative role, acting almost entirely on the advice of the government. However, he commanded great moral authority as an essential symbol of the country's unity.

In 1969, Juan Carlos was named as General Franco's successor and was given the title of 'Prince of Spain'. Upon the death of Franco in 1975, Juan Carlos acceded to the throne of Spain. The current Spanish constitution refers to the monarch by the simple title "King of Spain". Aside from this title, the constitution allows for the use of other historic titles pertaining to the Spanish monarchy, without specifying them. This was also reiterated by a decree promulgated on 6 November 1987 concerning titles of members of the royal family. Since his abdication in 2014, King Juan Carlos has retained, by courtesy, the title and style of King that he enjoyed during his reign.

Iñaki Urdangarin, who married Juan Carlos' daughter Cristina in 1997, became embroiled in a financial scandal in 2011 and was convicted of tax evasion in 2017.

In July 2000, Juan Carlos was the target of an enraged protester when former priest Juan María Fernández y Krohn, who had once attacked Pope John Paul II, breached security and attempted to approach the king.

Juan Carlos also hunts bears; in October 2004, he angered environmental Activists by killing nine bears, one of which was a pregnant female, in central Romania. It was alleged by the Russian regional authorities that in August 2006 Juan Carlos shot a drunken tame bear (Mitrofan the Bear) during a private hunting trip to Russia; the Office of the Spanish Monarchy denied this claim.

When the media asked Juan Carlos in 2005 if he would endorse the bill legalising same-sex marriage that was then being debated in the Cortes Generales, he answered "Soy el Rey de España y no el de Bélgica" ("I am the King of Spain, not of Belgium") – a reference to King Baudouin of Belgium, who had refused to sign the Belgian law legalising abortion. The King gave his Royal Assent to Law 13/2005 on 1 July 2005; the law legalising same-sex marriage was gazetted in the Boletín Oficial del Estado on 2 July, and came into effect on 3 July.

In November 2007, at the Ibero-American Summit in Santiago de Chile, during a heated exchange, Juan Carlos interrupted Venezuelan President Hugo Chávez, saying, "¿Por qué no te callas?" ("Why don't you shut up?"). Chávez had been interrupting the Spanish Prime Minister, José Luis Rodríguez Zapatero, while the latter was defending his predecessor and political opponent, José María Aznar, after Chávez had referred to Aznar as a fascist and "less human than snakes". The King shortly afterwards left the hall when President Daniel Ortega of Nicaragua accused Spain of intervention in his country's elections and complained about some Spanish Energy companies working in Nicaragua. This was an unprecedented diplomatic incident and a rare display of public anger by the King.

After the King's son-in-law Iñaki Urdangarín was accused of corruption (the "Urdangarín affair"), the King in 2011 for the first time detailed the yearly royal budget of 8.3 million euros, excluding expenses such as the electricity bill, paid by the State.

In April 2012, the King underwent surgery for a triple fracture of the hip at the San Jose Hospital, Madrid, following a fall on a private elephant-hunting trip to Botswana. He also underwent a hip operation in September 2013 at Madrid's Quirón hospital.

The Spanish public also gave a broadly positive opinion not only of the abdication but of his reign as a whole. According to a poll taken by El Mundo, 65 percent saw the king's reign as either good or very good, up from 41.3 percent. Overall, 55.7 percent of those polled in the 3–5 June survey by Sigma Dos supported the institution of the monarchy in Spain, up from 49.9 percent when the same question was posed six months prior. 57.5 percent believed the Prince could restore the royal family's lost prestige. An overwhelming majority of Spaniards believed the new king, Felipe VI, would make a good monarch and more than three-quarters believed King Juan Carlos had been right to hand over the throne to his son.

Spanish news media speculated about the King's Future in early 2014, following criticism and family scandal; the King's chief of staff in a briefing denied that the 'abdication option' was being considered. On the morning of 2 June 2014, Prime Minister Mariano Rajoy made a televised announcement that the King had told him of his intention to abdicate. Later, the King delivered a televised address and announced that he would abdicate the throne in favour of the Prince of Asturias. Royal officials described the King's choice as a personal decision which he had been contemplating since his 76th birthday at the start of the year. The King reportedly said, "[I] don't want my son to grow old waiting like Prince Charles." As required by the Spanish constitution, any abdication would be settled by means of an organic law. A draft law was passed with 299 in favour, 19 against and 23 abstaining. On 18 June, he signed the organic law passed by parliament several hours before his abdication took effect. Felipe was enthroned on 19 June 2014, and his granddaughter Leonor became the new Princess of Asturias.

A benign 17-19mm tumour was removed under general anaesthetic from King Juan Carlos’ right lung in an operation carried out in the "Hospital Clínic" of Barcelona in May 2010. The operation followed an annual check-up, and Juan Carlos was not expected to need any further treatment.