 As a young athlete, Sewell played fly-half at rugby and was twice selected to play for England Schools' Under 15 team.On leaving school, he first became a sprinter with Blackheath Harriers, and then signed as a professional footballer for Bexleyheath & Welling in 1954.A key man at centre-half in the Kent League team at Bexleyheath, Sewell was transferred to Charlton Athletic on 5 January 1955.