Alexa Chung was raised in Privett, Hampshire, England by her three-quarters Chinese and one-quarter English father, Philip, a graphic designer, and her English mother, Gillian (née Burgess), a housewife. She is the youngest of four children and has two brothers, James "Jamie" and Dominic, and one sister, Natalie Chung.

She attended the local comprehensive school, Perins College, and later the sixth form Peter Symonds College, Winchester (2000–2002). She had been accepted by King’s College London to read English and by Chelsea College of Art and Design to do an art foundation course, with two 'A' grades (English and Art) and a 'B' (History) at A-level, before being scouted by a modelling agency.

Chung was approached in the comedy tent at Reading Festival at the age of 16 by "Jane Duval, the influential British scout " and was taken on by the London-based Storm Model Management.

She modelled for teen magazines such as Elle Girl and CosmoGIRL! and appeared in adverts for products including Fanta, Sony Ericsson, Sunsilk and Tampax. She also modelled for Urban Outfitters and Shiatzy Chen. She appeared in music videos for artists including The Streets, Westlife, Delta Goodrem, Reuben and Holly Valance and starred as Jake in a scripted reality show called Shoot Me, broadcast on Fashion TV in 2005.

After four years, Chung quit modelling with the intention of beginning an art foundation or fashion journalism course, having become disillusioned. She had developed a “distorted body image” and “low self-esteem” through modelling.

After becoming a TV personality, Chung made a return to occasional modelling. In 2008, she was the face of both Australian fashion label Antipodium's SS 2008 collection and Oxfam's ethical fashion range, Revamped. In September 2008, Chung walked the catwalk as part of the Vivienne Westwood Red Label Spring/Summer 2009 show at London Fashion Week. In early 2009, she became the face of British high street giant New Look. Chung then joined London-based modelling agency, Select, and, in April 2009, she modelled Wren's Holiday 2009 collection, alongside her friend Tennessee Thomas.

In July 2009, Chung moved to Next Model Management and became the face of DKNY Jeans.

In January 2010, she became the main model of the SS10 Pepe Jeans London campaign.

During February 2010 Chung documented her daily fashion and style choices in the VOGUE.COM feature Today I'm Wearing, which showcases the wardrobes of budding style icons.

Chung appeared on the cover of the May 2009 issue of Harper's Bazaar UK wearing a Giambattista Valli gown, and later on the cover of the March 2010 issue of UK Vogue wearing Ralph Lauren.