He was educated at Howard Gardens High School, Cardiff and at the London School of Economics where he studied law. He served in the RAF during the Second World War. He was in Cairo in 1944 when some of the British military personnel stationed there set up a “Forces Parliament” in which they debated the structure of society they wanted to see in the post-war world. He worked as a journalist in Italy after the war and subsequently as a solicitor. In 1951 he established his own law firm, Leo Abse & Cohen at Cardiff, which eventually grew to be the largest in the city. He soon came to enjoy a burgeoning reputation as the most effective miners' lawyer, and in 1986, he was the first solicitor to be granted audience to the High Court.

Leo Abse had joined the Labour Party in 1934. He worked as a recruiter for the Republican cause during the Spanish Civil War. He was elected as Chairman of Cardiff Labour Party for two years from 1951 to 1953 and he was a Labour member of the Cardiff City Council, 1954-59. In both capacities he made the Cardiff MPs, James Callaghan and George Thomas, uncomfortable from time to time.

Abse fought the then-safe Conservative seat of Cardiff North in the 1955 general election, but, as widely expected, was predictably heavily defeated. When D. G. West was raised to the peerage, he served as the Labour MP for Pontypool from a by-election in 1958 until 1983, and subsequently for Torfaen until his retirement from parliament in 1987. In the 1958 by-election, Abse romped home with 20,000 votes, to the Conservative 6,273, and the Liberal 2,927. Huge majorities ensued thereafter until he retired.

He was the chairman of the Welsh Parliamentary Party from 1976 until 1987, and also served as chair of the Welsh Parliamentary Labour Group. He was also a long-term member of the advisory council of the British Humanist Society. He was a prominent campaigner against capital punishment in 1969 and one of the more prominent of the five Welsh Labour MPs who campaigned strenuously against devolution during the run-up to the referendum on 1 March 1979. He never held office, nor indeed wished to.

Abse was briefly Chairman of the Welsh Affairs Select Committee when it was first set up in 1979, but resigned in November 1981. One of the reasons why he opposed devolution was that he thought some in Wales, whom he called “fanatics”, wanted to use it to promote the use of the Welsh language. He opposed in particular proposals for criminal juries comprising only Welsh speakers, and described Welsh-language television as an “expensive farce” and a “gravy train”.

In the House of Commons Abse swiftly acquired a reputation for independence of spirit. He made a point of dressing flamboyantly on Budget Day, and liked to drop references from Freudian psychotherapy into his speeches. Although his abilities might have taken him to high office, Abse remained a backbench MP. This factor, together with the fact that he had a safe seat, freed him from the restrictions that prevented most other MPs from taking up controversial subjects.

No backbench MP could exceed his success in getting Private Member's Bills on to the statute book. During a notably long and colourful parliamentary career as a backbench Labour MP, he acted either as sponsor or co-sponsor of a formidable array of private members' acts on divorce, homosexuality, family planning, legitimacy, widows' damages, industrial injuries, congenital disabilities, and children. It was his Sexual Offences Bill, that received royal assent in July 1967, a full decade after the Wolfenden Report, that finally decriminalised homosexual activity between consenting adults. He was also a dominant figure behind the Divorce Reform Act 1969, which liberalised the laws on divorce. Abse himself deserves to be remembered as one of the most significant social reformers of 20th-century Britain.

Abse added to his reputation for taking maverick stances by strongly urging that British forces be withdrawn from Northern Ireland. He opposed nuclear power and nuclear weapons, and criticised Margaret Thatcher for insisting that Argentina unconditionally surrender over the Falkland Islands. However, he supported British membership of the European Communities. At the same time he occupied a large number of public offices, including serving as a member of the court of the National Museum of Wales and of the University of Wales, in both instances from 1981 until 1987. He was Regents Lecturer at the University of California in 1984.

His many publications include the volumes Private Member (1973), Margaret, Daughter of Beatrice (1990) and Tony Blair: the Man behind the Smile (2002). Each book from Abse was regarded as an exercise in outdoing its predecessor in outrageousness. Abse's approach was to study the relationship between politics and personality. He dared to place under scrutiny the interior, personal lives of practising politicians. Small wonder that Leo Abse made enemies - exclusively the powerful, particularly prime ministers. Among his many interests were Italian wines, Cuban cigars and numerous visits to various art galleries throughout Europe.

Leo was married 1955 to Marjorie Davies who died in 1996. They had two children Tobias and Bathsheba. He married for a second time in the year 2000 Ania Czepulkowska, a textile designer from Gdansk, when he was 83 and she was 33. Abse's friends were hugely grateful to her for looking after him in his deaf, great old age. He died 19 August 2008. He left his £1.2m estate to Ania, disinheriting his children and grandchildren.

A bust of Abse was unveiled at the National Museum of Wales in Cardiff on 22 October 2009. The sculpture was funded by Leo Abse and Cohen, the firm of solicitors that he founded, and was made by Abse's second cousin, Luke Shepherd. A large group of his political papers is in the custody of the Welsh Political Archive at the National Library of Wales.