Egerton attended Ditcham Park School near Petersfield in Hampshire, where she says she was bullied. She also states she truanted, preferring to go to modeling shoots. After finishing school at 16 with nine A-grade GCSEs, Egerton decided to pursue acting rather than going to university. She had a brief job as a babysitter for three children. She previously shared a flat in Marylebone, Central London, with her St. Trinian's (2007) castmate, Talulah Riley, whom she described as her "soulmate". She lived in Hampstead with theatre producer Jamie Hendry.

She has cited Takeshi Kitano as her favourite film director.