The "comeback king" of British politics, who has resigned twice from Cabinet posts, was awarded the titles of First Secretary of State, Secretary of State for Business, Innovation and Skills and Lord President of the Council in last week's reshuffle, a much-expanded role.
Nicknamed "The Prince of Darkness" by satirical magazine Private Eye, he is one of seven unelected peers who are entitled to attend Cabinet.
There is speculation that the reshuffle has created a rift between Lord Mandelson and Ed Balls, the Schools Secretary, who is said to blame him for missing out on the job of Chancellor.
Lord Mandelson is the grandson of Labour minister Herbert Morrison. He was born in 1953 and read philosophy, politics and economics at St Catherine's College, Oxford.
He became Labour's director for campaigns and communications in 1985, and was elected MP for Hartlepool in 1992. A close ally of Tony Blair, he was made Minister without Portfolio in the Cabinet Office in 1997, later acquiring responsibility for the Millennium Dome. He oversaw a boost in science spending as Secretary of State for Trade and Industry, but in 1998 resigned over a secret loan he received from Cabinet colleague Geoffrey Robinson.
Mr Blair brought him back a year later as Northern Ireland Secretary, but in January 2001 he quit over misconduct allegations. He was later cleared of any wrongdoing.
"The only thing more unpopular among Labour MPs than top-up fees is Peter Mandelson," a Whitehall source said in 2003, in light of news that he had set up a Downing Street "war room" to push fees legislation through.
He spent four years as European Commissioner for Trade before Mr Brown made him Business Secretary and a life peer last year, ending a feud that had lasted since Lord Mandelson backed Mr Blair to become Labour Party leader.
Later the same month, he found himself embroiled in the Corfu "yacht-gate" affair, following reports that he had maintained contact with Russian oligarch Oleg Deripaska during a period in which he had cut aluminium tariffs, a move that benefited Mr Deripaska's company.
- Lord Drayson keeps his post as Science Minister but adds responsibility for research and procurement in the Ministry of Defence. David Lammy, Higher Education Minister, also has a ministerial post, as do Pat McFadden, Rosie Winterton, Kevin Brennan and Lord Davies of Abersoch.
Junior ministers are Lord Carter of Barnes, Ian Lucas, Baroness Vadera and Lord Young of Norwood Green. Details of their responsibilities were unavailable as Times Higher Education went to press.