The world and his wife attended the party, which took place at the birthday girl's new country estate, a priory in Oxfordshire, which she has recently bought with her husband, Matthew Freud.
From adland, WPP's Sir Martin Sorrell, the Omnicom vice-chairman, Peter Mead, and CHI & Partners' Johnny Hornby partied away with the likes of David Cameron, David Miliband, Tony Blair and senior newspaper editors.
Given the tone of said conversation, Mandelson's return to the fold must have come after a conversion of Pauline proportions.
At one point, Blair and Miliband were spotted having a private and animated chat in a corner of the marquee. Oh to have been a fly on the canvas!
Brown was unable to attend (which is probably just as well), as he was otherwise engaged with the Queen at Balmoral. But he did send a birthday message.
Other faces spotted on the night were The Sun editor, Rebekah Wade, and her Times counterpart, James Harding.