Business Secretary Peter Mandelson took his seat in the House of Lords following his appointment to the Cabinet by Gordon Brown.
Baron Mandelson of Foy in the county of Herefordshire and Hartlepool in the county of Durham - who will be known simply as Lord Mandelson - wore the traditional ermine-lined scarlet robes as he swore allegiance to the Queen in a short ceremony of induction.
New peers are flanked by two supporters when they take their seat and Mr Mandelson choose Labour former Lords Leader Baroness Jay of Paddington and Labour former Lord Chancellor Lord Falconer of Thoroton.
Mr Mandelson, who twice resigned from Tony Blair's Cabinet, was surprisingly brought back by Mr Brown in his reshuffle this month.
The former Hartlepool MP was recalled from his position as EU Trade Commissioner to take charge of the Department for Business, Enterprise and Regulatory Reform and also to sit on the National Economic Council, a body designed by Mr Brown to fight the current global financial chaos.
Outside Number 10 when he was appointed, the former EU Trade Commissioner joked that it would be "third time lucky" and declared the present financial crisis called for "all hands on deck".
The man who is referred to by some as the Prince of Darkness, revealed he had consulted Tony Blair before accepting his job. The former Prime Minister told his friend the decision was a "no-brainer".
It signalled the end of a 14-year feud with Gordon Brown and Mr Mandelson said they were both part of the same "Labour family", adding: "When times get tough, families pull together and that's what we are doing."
