Montgomery was born in Kennington, Surrey, in 1887, the fourth child of nine, to an Ulster-Scots Church of Ireland minister, The Reverend Henry Montgomery, and his wife, Maud (née Farrar). The Montgomerys, an 'Ascendancy' gentry family, were the County Donegal branch of the Clan Montgomery. Henry Montgomery, at that time Vicar of St Mark's Church, Kennington, was the second son of Sir Robert Montgomery, a native of Inishowen in County Donegal in Ulster, the noted colonial administrator in British India, who died a month after his grandson's birth. He was probably a descendant of Colonel Alexander Montgomery (1686–1729). Bernard's mother, Maud, was the daughter of The V. Rev. Frederic william Canon Farrar, the famous preacher, and was eighteen years younger than her husband. After the death of Sir Robert Montgomery, Henry inherited the Montgomery ancestral estate of New Park in Moville in Inishowen in Ulster. There was still £13,000 to pay on a mortgage, a large debt in the 1880s, and Henry was at the time still only an Anglican vicar. Despite selling off all the farms that were at Ballynally, "there was barely enough to keep up New Park and pay for the blasted summer holiday" (i.e., at New Park).

It was a financial relief of some magnitude when, in 1889, Henry was made Bishop of Tasmania, then still a British colony and Bernard spent his formative years there. Bishop Montgomery considered it his duty to spend as much time as possible in the rural areas of Tasmania and was away for up to six months at a time. While he was away, his wife, still in her mid-twenties, gave her children "constant" beatings, then ignored them most of the time as she performed the public duties of the bishop's wife. Of Bernard's siblings, Sibyl died prematurely in Tasmania, and Harold, Donald and Una all emigrated. Maud Montgomery took little active interest in the education of her young children other than to have them taught by tutors brought from Britain. The loveless environment made Bernard something of a bully, as he himself recalled, "I was a dreadful little boy. I don't suppose anybody would put up with my sort of behaviour these days." Later in life Montgomery refused to allow his son David to have anything to do with his grandmother, and refused to attend her funeral in 1949.

The family returned to England once for a Lambeth Conference in 1897, and Bernard and his brother Harold were educated for a term at The King's School, Canterbury. In 1901, Bishop Montgomery became secretary of the Society for the Propagation of the Gospel, and the family returned to London. Montgomery attended St Paul's School and then the Royal Military College, Sandhurst, from which he was almost expelled for rowdiness and violence. On graduation in September 1908 he was commissioned into the 1st Battalion the Royal Warwickshire Regiment as a second lieutenant, and first saw overseas Service later that year in India. He was promoted to lieutenant in 1910, and in 1912 became adjutant of the 1st Battalion of his regiment at Shorncliffe Army Camp.

The Great War began in August 1914 and Montgomery moved to France with his battalion that month, which was at the time part of the 10th Brigade of the 4th Division. He saw action at the Battle of Le Cateau that month and during the retreat from Mons. At Méteren, near the Belgian border at Bailleul on 13 October 1914, during an Allied counter-offensive, he was shot through the right lung by a sniper. Montgomery was hit once more, in the knee. He was awarded the Distinguished Service Order for gallant leadership: the citation for this award, published in the London Gazette in December 1914 reads:

After recovering in early 1915, he was appointed to be brigade major first of 112th Brigade and then with 104th Brigade under training in Lancashire. He returned to the Western Front in early 1916 as a general staff officer in the 33rd Division and took part in the Battle of Arras in April/May 1917. He became a general staff officer with IX Corps, part of General Sir Herbert Plumer's Second Army, in July 1917.

Montgomery served at the Battle of Passchendaele in late 1917 before finishing the war as GSO1 (effectively Chief of Staff) of the 47th (2nd London) Division, with the temporary rank of lieutenant colonel. A photograph from October 1918, reproduced in many biographies, shows the then unknown Lieutenant Colonel Montgomery standing in front of Winston Churchill (then the Minister of Munitions) at the parade following the liberation of Lille.

After the First World War Montgomery commanded the 17th (Service) Battalion of the Royal Fusiliers, a battalion in the British Army of the Rhine, before reverting to his substantive rank of captain (brevet major) in November 1919. He had not at first been selected for the Staff College in Camberley, Surrey (his only hope of ever achieving high command). But at a tennis party in Cologne, he was able to persuade the Commander-in-Chief (C-in-C) of the British Army of Occupation, Field Marshal Sir william Robertson, to add his name to the list.

The Wehrmacht's objectives for Betrieb Wacht am Rhein (Operation Watch on the Rhine) was to split the Allied Armies in two by attacking the center of the allied armies through the Ardennes Forest in Belgium (during one of the worst storms in history) and then turning north to recapture the port at Antwerp. On the north-western side of the battle area was Montgomery's 21st Army Group which anchored the northern flank of the allied lines, with Bradley's army group on Montgomery's right flank and Patton's 3rd Army on the far right of Bradley's flank.

In May 1923, Montgomery was posted to the 49th (West Riding) Infantry Division, a Territorial Army (TA) formation. He returned to the 1st Battalion, Royal Warwickshire Regiment in 1925 as a company commander and was promoted to major in July 1925. From January 1926 to January 1929 he served as Deputy Assistant Adjutant General at the Staff College, Camberley, in the temporary rank of lieutenant colonel.

In 1925, in his first known love affair, Montgomery, then in his late thirties, courted a 17-year-old girl, Miss Betty Anderson. His method of courtship apparently included drawing diagrams in the sand of how he would deploy his tanks and infantry in a Future war, a contingency which seemed very remote at that time. She respected his ambition and single-mindedness, but declined his proposal of marriage.

In 1927, he met and married Elizabeth (Betty) Carver, née Hobart, widow of Oswald Carver, Olympic rowing medallist who had been killed in the First World War. Betty Carver was the sister of the Future Second World War commander, Major General Sir Percy Hobart. Betty Carver had two sons in their early teens, John and Dick, from her first marriage. Dick Carver later wrote that it had been "a very brave thing" for Montgomery to take on a widow with two children. Montgomery's son, David, was born in August 1928.

In January 1929 Montgomery was promoted to brevet lieutenant colonel. That month he returned to the 1st Battalion, Royal Warwickshire Regiment again, as Commander of Headquarters Company; he went to the War Office to help write the Infantry Training Manual in mid-1929. In 1931 Montgomery was promoted to substantive lieutenant colonel and became the Commanding Officer (CO) of the 1st Battalion, Royal Warwickshire Regiment and saw Service in Palestine and British India. He was promoted to colonel in June 1934 (seniority from January 1932). He attended and was then recommended to become an instructor at the Indian Army Staff College (now the Pakistan Army Staff College) in Quetta, British India.

On completion of his tour of duty in India, Montgomery returned to Britain in June 1937 where he took command of the 9th Infantry Brigade with the temporary rank of brigadier. His wife died that year.

In 1938, he organised an amphibious combined operations landing exercise that impressed the new C-in-C of Southern Command, General Sir Archibald Percival Wavell. He was promoted to major general on 14 October 1938 and took command of the 8th Infantry Division in Palestine. There he quashed an Arab revolt before returning in July 1939 to Britain, suffering a serious illness on the way, to command the 3rd (Iron) Infantry Division. On hearing of the rebel defeat in April 1939, Montgomery said, "I shall be sorry to leave Palestine in many ways, as I have enjoyed the war out here".

Britain declared war on Germany on 3 September 1939. The 3rd Division was deployed to Belgium as part of the British Expeditionary Force (BEF). During this time, Montgomery faced serious trouble from his military superiors and the clergy for his frank attitude regarding the sexual health of his Soldiers, but was defended from dismissal by his superior Alan Brooke, commander of II Corps. Montgomery's training paid off when the Germans began their invasion of the Low Countries on 10 May 1940 and the 3rd Division advanced to the River Dijle and then withdrew to Dunkirk with great professionalism, entering the Dunkirk perimeter in a famous night-time march that placed his forces on the left flank, which had been left exposed by the Belgian surrender. The 3rd Division returned to Britain intact with minimal casualties. During Operation Dynamo — the evacuation of 330,000 BEF and French troops to Britain — Montgomery assumed command of the II Corps.

Montgomery was ordered to make ready his 3rd Division to invade the neutral Portuguese Azores. Models of the islands were prepared and detailed plans worked out for the invasion. The invasion plans did not go ahead and plans switched to invading Cape Verde island also belonging to neutral Portugal. These invasion plans also did not go ahead. Montgomery was then ordered to prepare plans for the invasion of neutral Ireland and to seize Cork, Cobh and Cork harbour. These invasion plans, like those of the Portuguese islands, also did not go ahead and in July 1940, Montgomery was appointed acting lieutenant-general, and placed in command of V Corps, responsible for the defence of Hampshire and Dorset, and started a long-running feud with the new Commander-in-Chief (C-in-C) of Southern Command, Lieutenant General Claude Auchinleck.

In April 1941, he became commander of XII Corps responsible for the defence of Kent. During this period he instituted a regime of continuous training and insisted on high levels of physical fitness for both officers and other ranks. He was ruthless in sacking officers he considered would be unfit for command in action. Promoted to temporary lieutenant-general in July, in December Montgomery was given command of South-Eastern Command overseeing the defence of Kent, Sussex and Surrey.

The Second Battle of El Alamein began on 23 October 1942, and ended 12 days later with one of the first large-scale, decisive Allied land victories of the war. Montgomery correctly predicted both the length of the battle and the number of casualties (13,500). Soon after Allied armoured units and infantry broke through the German and Italian lines and were pursuing the enemy forces at speed along the coast road, a violent rainstorm burst over the region, bogging down the tanks and support trucks in the desert mud. Montgomery, standing before his officers at headquarters and close to tears, announced that he was forced to call off the pursuit. Historian Corelli Barnett has pointed out that the rain also fell on the Germans, and that the weather is therefore an inadequate explanation for the failure to exploit the breakthrough, but nevertheless the Battle of El Alamein had been a great success. Over 30,000 prisoners of war were taken, including the German second-in-command, General von Thoma, as well as eight other general officers. Rommel, having been in a hospital in Germany at the start of the battle, was forced to return on 25 October 1942 after Stumme – his replacement as German commander – died of a heart attack in the early hours of the battle.

Montgomery was notorious for his lack of tact and diplomacy. Even his "patron," the Chief of the Imperial General Staff Lord Alanbrooke, frequently mentions it in his war diaries: "he is liable to commit untold errors in lack of tact" and "I had to haul him over the coals for his usual lack of tact and egotistical outlook which prevented him from appreciating other people's feelings". One incident that illustrated this occurred during the North African campaign when Montgomery bet Walter Bedell Smith that he could capture Sfax by the middle of April 1943. Smith jokingly replied that if Montgomery could do it he would give him a Flying Fortress complete with crew. Smith promptly forgot all about it, but Montgomery did not, and when Sfax was taken on 10 April he sent a message to Smith "claiming his winnings". Smith tried to laugh it off, but Montgomery was having none of it and insisted on his aircraft. It got as high as Eisenhower who, with his renowned skill in diplomacy, ensured Montgomery did get his Flying Fortress, though at a great cost in ill feeling. Even Alanbrooke thought it "crass stupidity".

After the war Lord Montgomery became the Commander-in-Chief (C-in-C) of the British Army of the Rhine (BAOR), the name given to the British Occupation Forces, and was the British member of the Allied Control Council. Montgomery was also President of Portsmouth Football Club between 1944 and 1961. He was created 1st Viscount Montgomery of Alamein in 1946. He was Chief of the Imperial General Staff (CIGS) from 1946 to 1948, succeeding Alanbrooke. As CIGS, Montgomery toured Africa in 1947 and in a secret 1948 report to Prime Minister Clement Attlee's government proposed a "master plan" to exploit the raw materials of Africa, thereby counteracting the loss of British influence in Asia. Montgomery held racist views towards Africans, describing them as "complete savages" incapable of developing their own countries. He was barely on speaking terms with his fellow chiefs, sending his VCIGS to attend their meetings and he clashed particularly with Sir Arthur Tedder, who was by now Chief of the Air Staff (CAS). When Montgomery's term of office expired, Prime Minister Attlee appointed Sir william Slim from retirement with the rank of field marshal as his successor; when Montgomery protested that he had told his protégé, General Sir John Crocker, former commander of I Corps in the 1944–45 North-West Europe Campaign, that the job was to be his, Attlee is said to have given the memorable retort "Untell him".

In August 1945, whilst Alanbrooke, Sir Andrew Cunningham and Sir Charles Portal were discussing their possible successors as "Chiefs Of Staff", they concluded that Montgomery would be very efficient as CIGS from the Army's point of view but that he was also very unpopular with a large proportion of the Army. Despite this, Cunningham and Portal were strongly in favour of Montgomery succeeding Alanbrooke after his retirement. Prime Minister Winston Churchill, by all accounts a faithful friend, is quoted as saying of Montgomery, "In defeat, unbeatable; in victory, unbearable."

After the war he became Commander-in-Chief of the British Army of the Rhine (BAOR) in Germany and then Chief of the Imperial General Staff (1946–1948). He then served as Deputy Supreme Commander of NATO in Europe until his retirement in 1958.

He twice met Israeli general Moshe Dayan. After an initial meeting in the early 1950s, Montgomery met Dayan again in the 1960s to discuss the Vietnam War, which Dayan was studying. Montgomery was harshly critical of US strategy in Vietnam, which involved deploying large numbers of combat troops, aggressive bombing attacks, and uprooting entire village populations and forcing them into strategic hamlets. Montgomery said that the Americans' most important Problem was that they had no clear-cut objective, and allowed local commanders to set military policy. At the end of their meeting, Montgomery asked Dayan to tell the Americans, in his name, that they were "insane". In retirement he publicly supported apartheid after a visit to South Africa in 1962, and after a visit to China declared himself impressed by the Chinese leadership.

He was chairman of the governing body of St. John's School in Leatherhead, Surrey, from 1951 to 1966, and a generous supporter. Lord Montgomery was an Honorary Member of the Winkle Club, a noted charity in Hastings, East Sussex, and introduced Sir Winston Churchill to the club in 1955.

Montgomery's memoirs (1958) criticised many of his wartime comrades in harsh terms, including Eisenhower. He was threatened with legal action by Field Marshal Auchinleck for suggesting that Auchinleck had intended to retreat from the Alamein position if attacked again, and had to give a radio broadcast (20 November 1958) expressing his gratitude to Auchinleck for having stabilised the front at the First Battle of Alamein. The 1960 paperback edition of his memoirs contains a publisher's note drawing attention to that broadcast, and stating that in the publisher's view the reader might reasonably assume from Montgomery's text that Auchinleck had been planning to retreat "into the Nile Delta or beyond" and pointing out that it had been Auchinleck's intention to launch an offensive as soon as the Eighth Army was "rested and regrouped". Montgomery was stripped of his honorary citizenship of Montgomery, Alabama, and was challenged to a duel by an Italian officer.

During a visit to the Alamein battlefields in May 1967, he bluntly told high-ranking Egyptian Army officers that they would lose any war with Israel, a warning that was shown to be justified only a few weeks later in the Six-Day War.

Montgomery died from unspecified causes in 1976 at his home Isington Mill in Isington, in the County of Hampshire, aged 88. After a funeral at St George's Chapel, Windsor, his body was buried in Holy Cross churchyard, in Binsted, Hampshire.

General Patton's 3rd Army, which was 90 miles (140 km) to the south, switched from its mission in order to turn north and fought its way through the severe weather and German opposition and broke through to Bastogne, the bad weather cleared so that the USAAF and the RAF could resume air assault operations against Nazi armoured divisions and the Wehrmacht ran out of petrol.

Montgomery's solution to the dilemma was to attempt to remain Commander of All Land Forces until the end of the war, so that any victory attained – although achieved primarily by American formations – would accrue in part to him and thus to Britain. He would also be able to ensure that British units were spared some of the high-attrition actions, but would be most prominent when the final blows were struck. When that strategy failed, he persuaded Eisenhower to occasionally put some American formations under the control of the 21st Army Group, so as to bolster his resources while still maintaining the outward appearance of successful British effort.

Since SHAEF believed the Wehrmacht was no longer capable of launching a major offensive, nor that any offensive could be launched through such rugged terrain as the Ardennes Forest — particularly during winter — the Ardennes was used as an area to send US divisions, which had recently fought and sustained severe casualties, in the Battle of Hürtgen Forest to regroup and refit. It was also used as a place where new units recently from the US were sent to get some field experience in a safe place.

In the aftermath of Market Garden, Montgomery made holding the Arnhem salient his first priority, arguing that the 2nd British Army might still be able to break through and reach the wide open plains of northern Germany, and that he might be able to take the Ruhr by the end of October. In the meantime, the First Canadian Army, which been given the task of clearing the mouth of the river Scheldt, despite the fact that in the words of Copp and Vogel "...that Montgomery's Directive required the Canadians to continue to fight alone for almost two weeks in a battle which everyone agreed could only be won with the aid of additional divisions". For his part, Field Marshal Gerd von Rundstedt, the German commander of the Western Front ordered General Gustav-Adolf von Zangen, the commander of 15th Army that: "The attempt of the enemy to occupy the West Scheldt in order to obtain the free use of the harbor of Antwerp must be resisted to the utmost" (emphasis in the original). Rundstedt argued with Hitler that as long as the Allies could not use the port of Antwerp, the Allies would lack the logistical capacity for an invasion of Germany.

In the inter-war years he commanded the 17th (Service) Battalion, Royal Fusiliers and, later, the 1st Battalion, Royal Warwickshire Regiment before becoming commander of 9th Infantry Brigade and then General Officer Commanding (GOC) 8th Infantry Division.

On 18 June, Montgomery ordered Bradley to take Cherbourg while the British were to take Caen by 23 June. In Operation Epsom, the British VII Corps commanded by Sir Richard O'Connor attempted to outflank Caen from the west by breaking through the dividing line between the Panzer Lehr and the 12th SS to take the strategic Hill 112. Epsom began well with O'Connor's assault force, the British 15th Scottish Division breaking through and with the 11th Armoured Division stopping the counter-attacks of the 12th SS Division. General Friedrich Dollmann of the 7th Army had to commit the newly arrived II SS Corps to stop the British offensive. Dollmann, fearing that Epsom would be a success, committed suicide and was replaced by SS Oberstegruppenführer Paul Hausser. O'Connor, at the cost of about 4,000 men, had won a salient 5 miles (8.0 km) deep and 2 miles (3.2 km) wide but placed the Germans into an unviable long-term position. There was a strong sense of crisis in the Allied command as the Allies had advanced only about 15 miles (24 km) inland, at a time when their plans called for them to have already taken Rennes, Alençon and St. Malo. After Epsom, Montgomery had to tell General Harry Crerar that the activation of the First Canadian Army would have to wait as there was room at present in the Caen sector only for the newly arrived XII Corps under Lieutenant General Neil Ritchie, which caused some tension with Crerar who was anxious to get into the field. Epsom had forced further German forces into Caen but all through June and the first half of July Rommel, Rundstedt, and Hitler were engaged in planning for a great offensive to drive the British into the sea; it was never launched and would have required the commitment of a large number of German forces to the Caen sector.

The fewer the number of combat-experienced divisions the British had left at the end of the war, the smaller Britain’s influence in Europe was likely to be, compared to the emerging superpowers of the USA and the USSR. Montgomery was thus caught in a dilemma – the British Army needed to be seen to be pulling at least half the weight in the liberation of Europe, but without incurring the heavy casualties that such a role would inevitably produce. The 21st Army Group scarcely possessed sufficient forces to achieve such a military prominence, and the remaining divisions had to be expended sparingly.