Harold Sly was born in Appley Bridge, Lancashire in 1904. He played football for the Rover Company’s team before joining Birmingham, however, he never started a match for the club in the English Football League and dropped back into the amateur game with Tamworth Castle.
In 1927 he joined Gillingham of the Football League Third Division where he spent two seasons but never secured a regular place in the club’s first team, He then moved to Brighton & Hove Albion where he made 24 appearances between 1929 and 1933 before joining French club FC Sete.