They're not the most likely of friends – she's a Hollywood star while he tends to stay out of the limelight.
But the Duchess of Cambridge's brother James Middleton turned up to celebrate Mischa Barton’s new venture as she launched her first clothing store in London's Spitalfields area.
James joined tuxedo-clad Mischa, her sister Zoe and actor boyfriend Sebastien Knapp at the swanky bash.
The former OC star was channelling Hollywood chic in a tapered tuxedo and bright orange lipstick.
But he more than held his own next to the 26-year-old, looking trim and healthy in a smart casual outfit of a suit jacket and tight-fitting jeans.
His toned physique is the result of running – he once told HELLO! that he jogs six miles a day with his dog.
And the youngest Middleton combines his love of sport with charity ventures – earlier this year he joined his other sister Pippa, with whom he shares a £1 million house in Chelsea, on a 56 mile cross-country ski.
While James may not be quite as knowledgeable about clothes as his fashion-foward siblings, he will definitely have been interested in Mischa's plans to expand her business.
The 25-year-old, who was invited by Kate to give a reading at her wedding to Prince William, owns four businesses.
His most successful venture so far, entitled Cake Kit Company, has been nominated for several awards and won the prestigious Smarta 100 and Haines Watts Young Entrepeneur awards.
"I've won awards for my own efforts because I am young and good in my own right," the businessman told HELLO!.
"It's not just because of my family that I’ve been getting attention. I want to be known for my own contribution, not for being a 'relative of' – that's boring."
Cake Kit Company – with the slogan 'Home made, made even easier' – provides the cake mixture, icing, candles and even a disposable baking tray to make cakes shaped as football pitches, dinosaurs, rockets, fairies and handbags.
Like James Mischa has big plans for her brand. The boutique in Spitalfields area of the capital follows on from her successful handbag line, now in its ninth season, and she will also produce a make up line too.