A member of a local theatre group since she was 10, she was picked for her role in EastEnders when she was 16 and first appeared on the show in September 2000. She left the series in June 2005 and has since stated that she does not intend to return to EastEnders.
During summer 2005, Ryan appeared in a run of Who's the Daddy? at the King's Head Theatre. The play by Toby Young and Lloyd Evans is based on the David Blunkett paternity case.
Ryan had a small role in an episode of Agatha Christie's Marple which screened in February 2006, and also appeared in a small independent film the same year, Cashback.
Prior to Bionic Woman, she appeared in Jekyll for the BBC, a modern-day version of Strange Case of Dr Jekyll and Mr Hyde starring James Nesbitt. It began its six-episode run on 16 June 2007, and was released on DVD in the United States on 18 September 2007, one week prior to her debut in Bionic Woman.
Ryan also filmed an ITV romantic drama, Mister Eleven in 2008.
Also in 2009, Ryan filmed 126.96.36.199., a crime thriller starring Emma Roberts, Noel Clarke, Tamsin Egerton and Ophelia Lovibond.
In October 2010, she completed filming the terrorist thriller Cleanskin. That same year, she appeared in the comedy film Huge alongside Noel Clarke, Johnny Harris, Ralph Brown, Thandie Newton, and Tamsin Egerton, directed by Ben Miller. Another film, No Ordinary Trifle, is in post-production. Also in 2011, Ryan played Helena from A Midsummer Night's Dream in the BBC Learning project, Off By Heart Shakespeare and delivered the speech: "Lo, she is one of this confederacy!"
For the final months of 2012, Ryan toured with Will Young in a production of Cabaret. This production also played in London's West End.