Boateng was born in London of mixed Ghanaian and Scottish heritage, but lived in Ghana, where his father became a cabinet minister under Kwame Nkrumah, until the 1966 coup that ousted Nkrumah. After graduating from the University of Bristol he became a lawyer (originally a solicitor, though he later retrained as a barrister) specialising in civil rights cases. He was elected to the Greater London Council in 1981 as a member of Labour's left wing and a supporter of Ken Livingstone. As chair of the GLC's police committee, he was an outspoken critic of police relations with ethnic minorities.
In 1987 he was elected to the House of Commons. Like many other members of the 1980s left he became more moderate under the leadership of Neil Kinnock, who made him a junior spokesman in 1989. In 1992 he became shadow minister for the Lord Chancellor's department, a post he held until the 1997 general election.
With Labour's victory, Boateng became the UK's first black government minister, albeit in the surprisingly junior position of Parliamentary Under Secretary of State at the Department of Health. In 1998 he became a minister at the Home Office. Now a loyal supporter of the New Labour project, he defended the police and criticised his former GLC colleague Ken Livingstone's mayoral campaign.
In 2001 he was made Financial Secretary to the Treasury, and stepped up to become Chief Secretary and a member of the Cabinet in May 2002.