Máirtín Ó Muilleoir was born in Belfast in 1959. He attended St Mary's Christian Brothers Grammar School and graduated from Queen's University Belfast. In 1997, Ó Muilleoir became the part-owner of the Andersonstown News, subsequently the Belfast Media Group, of which Ó Muilleoir remains a director.

Ó Muilleoir was elected to Belfast City Council in 1987, representing Sinn Féin. He was re-elected in 1989 and 1993 and stood down in 1997 to focus on his business interests. Ó Muilleoir was re-elected to Council in 2011 and served until 2014.

In 2014, Ó Muilleoir was co-opted as a member of the Northern Ireland Assembly. He was elected to the Assembly in 2016 and was appointed Minister of Finance in the Northern Ireland executive.