The Smile hitmaker worked with Ronson on her 2006 album Alright, Still, and they teamed up again at the The Other Ball event in London's Mayfair.
Ronson was hosting the bash, which raised money for Arms Around The Child, an organisation which helps provide the basics for a child to be happy and healthy.
Allen performed at the event, and Florence Welch also took to the stage after banishing herself to the studio in recent months to work on a new album.
She tells the London Evening Standard, "The daily routine involves a bicycle and an anorak. I make a packed lunch - I do. Bicycle, anorak, packed lunch. It's kind of like, really old English. I've just really dedicated myself to one thing and everything else has got simplified."