Lewis began his career with Norwich City but did not make a competitive appearance for them, serving loans with Stockport County and Morecambe. In early 2008 he moved to Peterborough United, and was the regular goalkeeper during four seasons in which the club were promoted twice, relegated then promoted again. He lost his place in the team, and in 2012 moved to Cardiff City where he had a backup role and went out on loan to Blackpool and Fulham. In 2016 he moved to Scotland with Aberdeen, being installed as first choice and playing in three cup finals in his first three seasons; he was named captain in 2019.