Donaldson was born in Hammersmith, London, and educated at the Camden School for Girls, living in Kentish Town with her parents. Since being scouted by Select Model Management in 2003 at the age of 16 while shopping in Camden, Donaldson has landed campaigns for fashion houses such as Dior, Jil Sander, and Dolce & Gabbana and is the face of British luxury goods house Burberry. In 2009, Vogue Paris declared her one of the top 30 models of the 2000s.