The One Direction star joined fellow celebrities including Kelly Osbourne, Nick Grimshaw and Daisy Lowe to help Bob Geldof's daughter mark her big day.
Harry was snapped leaving China Tang at the posh Dorchester Hotel in the capital on Tuesday night (17 September), looking dapper in his shirt and jacket.
Earlier in the night, Pixie had uploaded a picture of Harry, Kelly, Nick, Daisy and a pal posing together over dinner.
Pixie's birthday evening also included a round of karaoke. She shared a picture of Kelly and Jack Guinness belting out a tune (apparently 'Total Eclipse of the Heart')!
But there's no word on whether Brit Award-winning Harry took a whirl on the mic... perhaps it would be too like work for him?!
Harry's night out came after he was recently seen on the front rows at London Fashion Week. Sadly for any Harry fans it seems he's ruled out any chance of a modelling career himself.
He told reporters backstage: "I don't know if I can walk in a straight line that well. I might fumble around, so maybe not."
Oh well, considering One Direction is pretty much THE biggest band on the planet and have a huge stadium tour coming up, we don't think he'll be missing things to do!
Watch One Direction's video for 'Best Song Ever' below: