Egerton started her acting career age six, following her older sister, Sophia to a local youth theatre, saying; "The whole reason I'm acting now is because I wanted to be doing what my big sister was doing". A year later, she appeared in a Royal Shakespeare Company musical production of The Secret Garden, playing Mary.

Egerton's other film credits include Sarah in Driving Lessons (2006) alongside Rupert Grint, and Flora in Knife Edge (2008). Her role as Holly Goodfellow in the 2005 film Keeping Mum sparked controversy as she appeared topless at the age of 16. She was to appear as Katrina in Eragon, but the scenes which she featured in were removed from the final cut.