Leo Abse was the son of Rudolf Abse, a Jewish solicitor and cinema owner who was born and lived in Cardiff, Wales. His younger brother Dannie Abse is a well known poet; his older brother Wilfred Abse (1915-2005) was an eminent psychoanalyst. Abse attended Howard Gardens High School in Cardiff followed by the London School of Economics where he studied law. Having joined the Labour Party in 1934, he clandestinely visited Spain during the closing months of the Spanish Civil War in 1939.
During World War II Abse served in the Royal Air Force. He was serving in Cairo in 1944 when the British forces stationed there set up a 'Forces Parliament' in which they debated the structure of society they wanted to see in the post-war world. Abse's idealistic left-wing views were fully in tune with the majority opinion among the lower ranks but the existence of the Parliament disturbed the more senior officers; when Abse moved a motion to nationalise the Bank of England he was arrested and the Forces Parliament was forcibly dissolved.
After the end of the war, Abse set up in practice as a solicitor in Cardiff. By 1951 he had a sufficiently good relation to establish his own law firm, Leo Abse and Cohen, which eventually grew to be the biggest in the City. He was also elected as Chairman of Cardiff Labour Party for two years from 1951, giving up the post when he was elected to Cardiff City Council. Abse fought the then safe Conservative seat of Cardiff North in the 1955 general election.
Backbench Labour MP for thirty-nine years, he famously got more legislation on to the statute book than any other backbencher. It was he who piloted through the House of Commons the 1967 Act partially decriminalising male homosexual acts.
In 1957 the Wolfenden report had recommended that the law be changed to legalise consenting male homosexual sex, but the government had taken no action. Based on his knowledge of psychiatry, Abse could not understand the reason for prohibiting gay sex and began to promote a Bill to put the Wolfenden recommendations into law in February 1962. He kept pressing the issue, and when Humphry Berkeley (Conservative MP for Lancaster) lost his seat in the 1966 general election Abse became the main sponsor for the legalisation. Although with the Labour landslide there was a majority for the Bill it was still vulnerable to procedural tricks; Abse managed to outwit opponents and persuaded Roy Jenkins to give the measure government time, which eventually saw the Bill through onto the statute book.
Abse's views on homosexuality were strongly influenced by his knowledge of psychotherapy. He argued that an obsession with the question of punishment of homosexuals "has hitherto prompted us to avoid the real challenge of preventing little boys from growing up to be adult homosexuals. Surely, what we should be pre-occupied with is the question of how we can, if it is possible, reduce the number of faulty males in the community." Abse put his arguments in this way partly to ensure that those MPs who were inclined to vote for the Bill did not feel they were endangering their masculinity but mostly because he had a view that "those who do not procreate are deprived or stunted" (the analysis of Antony Grey, who was leading the lobbying efforts of the Homosexual Law Reform Society and worked closely with him).
Other issues taken up by Abse included ending capital punishment. During the Six Day War he made a passionate attack on those Labour MPs who had supported the Arab cause. In 1968 he was appointed to a Home Office advisory committee on the penal system. As a sign of his popularity he was elected Chairman of the group of Welsh Labour MPs in 1971; he was already well enough known to write his autobiography Private Member in 1973.
In 2000 he married a Polish man, an electrician aged 33. Leo Abse died eight years later, aged 91, in the Charing Cross Hospital, London.