William Pitt Cassidy was born on 4 October 1940. He made 174 English football league appearances for Rotherham United, Brighton & Hove Albion and Cambridge United. He also played one Scottish league match match as a trialist for Hamilton Academical, and spent the 1968 North American Soccer League season with the Detroit Cougars.
Bill played 118 matches for the Seagulls between 1962 and 1967 and scored 25 goals.