The son of Sir Mortimer Margesson, he grew up in Worcestershire and was educated at Harrow School. and Magdalene College, Cambridge. He did not complete his degree, choosing instead to seek his fortune in the United States of America; but the First World War intervened.
Margesson served as an adjutant in the 11th Hussars. After the war he entered politics at the suggestion of Lord Lee of Fareham. In the 1922 general election he was elected as Member of Parliament for Upton.
Very soon after his election he was appointed Parliamentary Private Secretary to the Minister of Labour, Anderson Montague-Barlow. In the 1923 general election he lost his seat, but at the 1924 general election he returned to Parliament for Rugby, the seat for which he would sit for the next eighteen years, defeating the future Liberal National leader Ernest Brown in the process.
Margesson was appointed as an Assistant Government Whip, then two years later he became a more senior whip with the title Junior Lord of the Treasury. He held this until the Conservatives' defeat in the 1929 general election and in August 1931 he was reappointed to the same position upon the formation of the National Government. Following the November 1931 general election he was promoted to the senior position of Government Chief Whip.
Margesson's position was in many ways unprecedented, having the task of keeping in power a grouping composed of the Conservatives, National Labour and two groups of Liberals - the official Liberal Party and the Liberal National Party all behind a single government that sought to stand above partisan politics. With the government as whole commanding the support of five hundred and fifty-six MPs, as opposed to just fifty-eight opposition members, his main task was to ensure that the government stayed together and was able to pass contentious legislation without risking a major breach within the government. In several areas this proved tricky as different sections of the National combination came to denounce areas of government policy.
Margesson adopted a method of strong disciplinarianism, combined with selective use of patronage and the social effect of ostracism to secure every vote possible. An example of his methods was his letter to the House of Commons' youngest member, the future minister John Profumo, after the Norway Debate, in which Profumo had opposed his fellow-Tories. Margesson's letter included the following extraordinary passage: 'And I can tell you this, you utterly contemptible little shit. On every morning that you wake up for the rest of your life you will be ashamed of what you did last night.' On this occasion his methods failed.
Despite such behaviour, Margesson remained a much liked individual, with many members expressing personal admiration for him. Away from his duties he was known to be quite sociable; and within the parliamentary party; few bore him ill.
A major faultline lay over the question of introducing protective tariffs on imports as a prelude to negotiating a customs union within the British Empire. This proposed policy had deeply divided the Conservatives over the previous thirty years, but by now they, along with most of the National Labour and Liberal National members of the government, had become in favour of the policy amidst the chaos of the Great Depression. However the Liberal Party remained committed to the principle of Free Trade and were deeply reluctant to compromise. Whilst the Liberals themselves barely commanded the support of thirty-three MPs, they were one of only two parties in the government with a long independent history and there were fears that their withdrawal would turn the National Government into a mere Conservative rump, something the National Labour Prime Minister Ramsay MacDonald wished to avoid. At one stage it was agreed that members of the cabinet would suspend the principle of collective responsibility and "agree to differ" on the issue of tariffs. Matters were complicated further by the question of Cabinet appointments. When the Liberal President of the Board of Education Sir Donald Maclean died, Margesson insisted that to appoint another Liberal merely on the basis of party balance would inflame tensions amongst Conservative MPs, potentially lead to a poor appointment and maintain an imbalance since the Liberals had one more Cabinet Minister than the Liberal Nationals (and the National Labour Lord Privy Seal Lord Snowden was increasingly siding with the Liberals on all key divisions, thus providing a surrogate), despite the latter having two more MPs. The appointment of the Conservative Lord Irwin upset the Liberals who could not secure an assurance that the next Cabinet vacancy would be filled by a Liberal.
In the summer of 1932 the Ottawa Agreement was negotiated between the Dominion Governments and Free Trade seemed a dead cause within government. In September the Liberals resigned their ministerial offices, though did not withdraw complete support for the government until the following November. However the National Government did not break up as the remaining National Labour and Liberal National elements remained in government.
The government also came under fire from the Diehard wing of the Conservative Party over plans to implement Indian Home Rule. This policy was widely felt to be a hangover from the previous Labour government and one that few Conservative governments would have implemented. Many believed that the plan was only being pursued because of a desire to prove the government's non-partisan credentials and Conservative leader Stanley Baldwin's determination to implement the policy. For some, the question of the success of the policy became a question of the survival of the National Government. Opponents to Indian Home Rule found several spokespersons, most notably Winston Churchill, and they harried the Government at every stage, culminating in a rebellion of nearly one hundred Conservative MPs voting against the third reading of the Bill — the single highest number of Conservatives to vote against a three line whip in the twentieth century. Despite this, the Bill passed overwhelmingly.
Margesson was retained as Chief Whip when Stanley Baldwin retook the Premiership in June 1935 and now had to face further ruptions in the party over foreign policy and other matters. The government's majority was cut to two hundred and fifty in the November 1935 general election and the following month the leaking of the proposed Hoare-Laval Plan to grant two-thirds of Abyssinia to invading Italy provoked outrage amongst Conservative MPs. Margesson's reading of the mood led to the Foreign Secretary Sir Samuel Hoare being dropped from the government to abate feelings and keep the government in power.
The late 1930s were a turbulent time within the National Government, with rebellions over foreign policy, over unemployment, over agriculture and other matters routinely threatened to rock the government. Margesson was instrumental in heading off many of these rebellions and performing damage limitation upon others. He was instrumental in warding these off for, first Baldwin, and then Neville Chamberlain. However a well of discontent with the government's foreign policy grew, especially after Britain entered the Second World War. Eight months into the conflict, severe reverses in the Norwegian campaign led to the two day "Norway Debate" of May 7 and May 8 1940 in which the government came under severe criticism from its own supporters and witnessed a massive rebellion on a motion of confidence. The government maintained a majority, but Margesson's soundings revealed that that majority was imperilled unless the political composition of the government was widened. When Chamberlain realised he was unable to achieve this he resigned and was succeeded by Winston Churchill.
Many were surprised that Churchill retained Margesson as Chief Whip, little realising that there was no personal animosity between the two and that Churchill would have had less regard for Margesson if he had not carried out his functions as Chief Whip. Margesson proved a useful buttress of support as Churchill consolidated his position in government and when at the end of 1940 the position of Secretary of State for War fell vacant, Margesson was promoted to it.
In this office Margesson proved competent and efficient, but when in February 1942 Britain suffered severe military setbacks, including the loss of Singapore, Churchill was forced to make changes to his ministerial team and find scapegoats for the disasters. Margesson was dropped and replaced by his own Permanent Under-Secretary Sir P.J. Grigg — an unprecedented move. Margesson was first told of the change by Grigg himself, but accepted his fate as necessary for the government's future. Later that year he was made Viscount Margesson and his political influence waned heavily. He subsequently worked in the City.