Michael O Leary was born on 8 June 1936 in Cork, the son of a publican. He was educated at Presentation College, University College Cork and Columbia University. He became involved in labour politics and was employed in the Irish Trade and General Workers Union (ITGWU). In 1965 he was elected to Dáil Éireann as a Labour candidate. He encouraged the party to take a more left-wing stance in its policies.
O Leary was strongly opposed to the idea of a coalition with Fine Gael, however after the 1969 general election he realised there was a need for a new approach. In 1973 he became Minister for Labour in the coalition government with Fine Gael. In 1977 he was narrowly defeated by Frank Cluskey for the leadership of the party. In 1979 O Leary was elected to the European Parliament. In 1981 Cluskey resigned as Labour leader and O Leary was elected unanimously to succeed him.
In the 1981-1982 O Leary became Tánaiste and Minister for Energy. Shortly after the fall of that government he resigned as leader of the party. He then joined Fine Gael and was elected to the Dáil as a candidate of that party. He move back to his native Cork and practised as a barrister. He failed to be elected in the 1992 general election. In 1997 he was appointed as a District Court Judge.