Bevan believed that the Second World War would give Britain the opportunity to create "a new society". He often quoted an 1855 passage from Karl Marx: "The redeeming feature of war is that it puts a nation to the test. As exposure to the atmosphere reduces all mummies to instant dissolution, so war passes supreme judgment upon social systems that have outlived their vitality." At the beginning of the 1945 general election campaign Bevan told his audience that his goal was to eliminate any opposition to the Labour programme: "We have been the dreamers, we have been the sufferers, now we are the builders. We enter this campaign at this general election, not merely to get rid of the Tory majority. We want the complete political extinction of the Tory Party, and twenty-five years of Labour Government."

In 1919, he won a scholarship to the Central Labour College in London, sponsored by the South Wales Miners' Federation. There, he spent two years studying economics, politics and history. He read Marxism at the college, developing his left-wing political outlook. Reciting long passages by william Morris, Bevan gradually began to overcome the stammer that he had had since he was a child.

Bevan was one of the founding members of the "Query Club" with his brother Billy and Walter Conway. The club started in 1920 or 1921 and they met in Tredegar. They would collect money each week for any member who needed it. The club intended to break the hold that the Tredegar Iron and Coal Company had on the town by becoming members of pivotal groups in the community.

Upon returning home in 1921, he found that the Tredegar Iron & Coal Company refused to re-employ him. He did not find work until 1924 and his employer, the Bedwellty Colliery, closed down only ten months later. Bevan then had to endure another year of unemployment. In February 1925, his father died of pneumoconiosis.

In 1926, he found work again, this time as a paid union official. His wage of £5 a week was paid by the members of the local Miners' Lodge. His new job arrived in time for him to head the local miners against the colliery companies in what would become the General Strike. When the strike started on 3 May 1926, Bevan soon emerged as one of the Leaders of the South Wales miners. The miners remained on strike for six months. Bevan was largely responsible for the distribution of strike pay in Tredegar and the formation of the Council of Action, an organisation that helped to raise money and provided food for the miners.

In 1928, Bevan won a seat on Monmouthshire County Council. With that success he was picked as the Labour Party candidate for Ebbw Vale (displacing the sitting MP), and easily held the seat at the 1929 General Election. In Parliament he soon became noticed as a harsh critic of those he felt opposed the working man. His targets included the Conservative Winston Churchill and the Liberal David Lloyd George, as well as Ramsay MacDonald and Margaret Bondfield from his own Labour party (he targeted the latter for her unwillingness to increase unemployment benefits). He had solid support from his constituency, being one of the few Labour MPs to be unopposed in the 1931 General Election and this support grew through the 1930s and the period of the Great Depression in the United Kingdom.

He married fellow Socialist MP Jennie Lee in 1934. He was an early supporter of the socialists in Spain and visited that country in 1938. In 1936 he joined the board of the new socialist newspaper Tribune. His agitations for a united socialist front of all parties of the left (including the Communist Party of Great Britain) led to his brief expulsion from the Labour Party in March to November 1939 (along with Stafford Cripps and C. P. Trevelyan). But, he was readmitted in November 1939 after agreeing "to refrain from conducting or taking part in campaigns in opposition to the declared policy of the Party".

He strongly criticised the British government's rearmament plans in the face of the rise of Hitler's Germany, saying to the Labour conference in autumn 1937:

When the government introduced voluntary national Service in December 1938, Bevan argued that Labour should demand the nationalisation of the armaments industry, support Republican Spain and sign an Anglo-Soviet pact in return for its support. When Labour supported the government's scheme with no such conditions, Bevan denounced Labour for imploring the people on recruiting platforms to put themselves under the leadership of their opponents. When conscription was introduced six months later, Bevan joined the rest of the Labour Party in opposing it, calling it "the complete abandonment of any hope of a successful struggle against the weight of wealth in Great Britain". The government had no arguments to persuade young men to fight "except merely in another squalid attempt to defend themselves against the redistribution of international swag".

In August 1939 came the Nazi–Soviet Pact. In Parliament Bevan argued that this was the logical outcome of the government's foreign policy. However at this time of national crisis he voted for the first time with the government. He wanted the war to be not just a fight against fascism but a war for socialism.

Bevan was critical of the leadership of the British Army which he felt was class bound and inflexible. After Ritchie's retreat across Cyrenaica early in 1942 and his disastrous defeat by Rommel at Gazala, Bevan made one of his most memorable speeches in the Commons in support of a motion of censure against the Churchill government. In this he said, "The Prime Minister must realise that in this country there is a taunt on everyone's lips that if Rommel had been in the British Army he would still have been a sergeant ... There is a man in the British Army who flung 150,000 men across the Ebro in Spain, Michael Dunbar. He is at present a sergeant...He was Chief of Staff in Spain, he won the Battle of the Ebro, and he is a sergeant." In fact, Dunbar had been recommended for a commission, but rejected it himself.

When Bevan was made a minister in 1945, he envisioned a sector of public housing that would provide people with the choice to live in owner occupation or the private sector:

Substantial bombing damage and the continued existence of pre-war slums in many parts of the country made the task of housing reform particularly challenging for Bevan. Indeed, these factors, exacerbated by post-war restrictions on the availability of building materials and skilled labour, collectively served to limit Bevan's achievements in this area. 1946 saw the completion of 55,600 new homes; this rose to 139,600 in 1947 and 227,600 in 1948. While this was not an insignificant achievement, Bevan's rate of house-building was seen as less of an achievement than that of his Conservative (indirect) successor, Harold Macmillan, who was able to complete some 300,000 a year as Minister for Housing in the 1950s. Macmillan was able to concentrate full-time on Housing, instead of being obliged, like Bevan, to combine his housing portfolio with that for Health (which for Bevan took the higher priority).

Bevan said at a party rally in 1948: "That is why no amount of cajolery, and no attempts at ethical or social seduction, can eradicate from my heart a deep burning hatred for the Tory Party that inflicted those bitter experiences on me. So far as I am concerned they are lower than vermin. They condemned millions of first-class people to semi-starvation." The comment inspired the creation of the Vermin Club by angry Conservatives; they attacked Bevan for years for the metaphor. Labour Party deputy leader Herbert Morrison complained that Bevan's attack had backfired, for his words "did much more to make the Tories work and vote... than Conservative Central Office could have done."

After Bevan left the Health ministry in 1951 he could never regain his level of success. For all his wit and brilliance, Bevan proved a difficult colleague and feuded with fellow Labour Leaders, using his strong political base as a weapon. Kenneth O. Morgan says, "Bevan alone kept the flag of left-wing socialism aloft throughout — which gave him a matchless authority amongst the constituency parties and in party conference."

In March 1952, a poorly prepared Bevan came off the worse in an evening Commons debate on health with Conservative backbencher Iain Macleod: Macleod's performance led Churchill to appoint him Minister of Health some six weeks after his debate with Bevan.

Out of office, Bevan soon exacerbated the split within the Labour Party between the right and the left. For the next five years, Bevan was the leader of the left wing of the Labour Party, who became known as Bevanites. They criticised high defence expenditure (especially for nuclear weapons), called for better relations with the Soviet Union, and opposed the party leader, Attlee, on most issues. According to Richard Crossman Bevan hated "the in-fighting which you have to do in politics.... He wasn't cut out to be a leader, he was cut out to be a prophet." In 1954, Gaitskell defeated Bevan in a hard fought contest to be the Treasurer of the Labour Party. In March 1955, when Britain was preparing for tests of its first hydrogen bomb, Bevan led a revolt of 57 Labour MPs and abstained on a key vote. The Parliamentary Labour Party voted 141 to 113 to withdraw the whip from him, but it was restored within a month due to his popularity.

After the 1955 general election, Attlee retired as leader. Bevan contested the leadership against both Morrison and Labour right-winger Hugh Gaitskell, but it was Gaitskell who emerged victorious. Bevan's remark that "I know the right kind of political Leader for the Labour Party is a kind of desiccated calculating machine" was assumed to refer to Gaitskell, although Bevan denied it (commenting upon Gaitskell's record as Chancellor of the Exchequer as having "proved" this). However, Gaitskell was prepared to make Bevan Shadow Colonial Secretary, and then Shadow Foreign Secretary in 1956. Bevan was as critical of the Egyptian dictator Colonel Nasser's seizure of the Suez Canal on 26 July 1956 as he was of the subsequent Anglo-French military response. He compared Nasser with Ali Baba and the Forty Thieves. He was a vocal critic of the Conservative government's actions in the Suez Crisis, noticeably delivering high-profile speeches in Trafalgar Square on 4 November 1956 at a protest rally, and criticising the government's actions and arguments in Commons on 5 December 1956. At the Trafalgar rally, Bevan accused the government of a "policy of bankruptcy and despair". Bevan stated at the Trafalgar rally:

In 1957, Bevan, Richard Crossman and the Labour Party's General Secretary Morgan Phillips sued The Spectator magazine for libel, after one of its Writers described them as drinking heavily during an Italian Socialist Party conference. The article wrote that the three men:

His last speech in the House of Commons, in the Debate of 3 November 1959 on the Queen's Speech, referred to the difficulties of persuading the electorate to support a policy which would make them less well-off in the short term but more prosperous in the long term.

In 1959, Bevan was elected as Deputy Leader of the Labour Party. In pain, he checked into a hospital at the end of 1959 to undergo surgery for an ulcer, but malignant stomach cancer was discovered instead. Bevan died in his sleep, at 4.10pm on 6 July 1960, at the age of 62 at his home Asheridge Farm, Chesham, Buckinghamshire. His remains were cremated at Gwent Crematorium in Croesyceiliog.

Born into a working-class family in South Wales, Bevan eventually emerged one of Wales' most revered politicians. In 2004, over forty years after his death, he was voted first in a list of 100 Welsh Heroes, having been credited for his contribution to the founding of the welfare state.