Mischa Barton is one of those people who can sling on an old T-shirt and make it look like the last word in stylish. Since she hit the big time in teen soap The OC, she has been photographed sporting the sort of quirky, vintage look that sets trends. Describing her style as boho with a bit of rock, a recent example saw her at a festival teaming a fringed suede jacket with hotpants. Being a beautiful 21-year-old size 10 helps.

She spent the first six years of her life in Hammersmith, the second of three daughters of Mancunian financier Paul Barton and his Irish photographer wife Nuala. She still remembers the traditional uniform, including boater, that she wore to her English prep school, and is proud of her roots. "I love the sense of humour my family has, and we still have full English breakfasts." Her father's job meant a move to New York, where an eight-year-old Barton was "discovered" by an agent at summer camp. Her first job was an advert for jam. Modelling and small film parts followed, before at 15 she landed the part in The OC of Marissa, a troubled rich girl who pouted her way round California with Uggs and Chanel bags. It was a fashion education. Yet in contrast to Marissa, Mischa says she can be out of the house in 20 minutes.

"I travel so much, I guess my style is a mix of influences from all over," she says. "I love going shopping. What girl doesn't?" Favourite labels include Chloé and Gucci. Stylist Rachel Zoe helps out for red-carpet dos, which Barton accepts are just part of the job, "but it is fun getting dressed-up sometimes". She has confessed that at times she'd rather be at home reading a book ­ possibly by Dostoevsky, her favourite author ­ or making lasagne.

At the moment she is spokeswoman for an initiative called Visa Swap. The idea is that people take their unwanted, or unwise, fashion purchases to a temporary shop in Knightsbridge, London, and swap them with other people.

Designers including Phoebe Philo, Alice Temperley and Philip Treacy have donated pieces, while Barton has given some Roberto Cavalli linen shorts, a vintage Eagles tour T-shirt, a pair of leather boots and a navy Chanel skirt.

"I love trading clothes with friends and it's a great way to get new looks while off-loading stuff you're bored of. I love the environmental side of it; nothing is wasted." Anything unsold goes to clothes-recycling charity TRAID (to which Barton donated her fee for the shoot).

She arrives at our shoot wearing a black cotton dress by hip label PPQ, and giggling over a text about an ex-boyfriend. (Recent exes, about whom she is tight-lipped, include oil heir Brandon Davis and musician Cisco Adler.) She has the insouciance of the seasoned model but says she enjoys modelling "when everyone is vibing creatively". With that she goes through the racks of clothes like the veteran she is, and gets ready for her close-up.

Times recommends

• How I lost the gift of joy for a while
• How to get Coleen Rooney's post-baby body
• The A-Z of winter style rules