Dennis Mortimer was born in Liverpool. He began his career with Coventry City, where he came through the ranks to make more than 200 first-team appearances as a midfielder. His displays for Coventry attracted the attention of several top clubs, and he joined Aston Villa for £175,000 on Christmas Eve 1975.
Dennis captained Villa to the 1980-81 Football League championship, the club’s first League title for more than 70 years. He then led the team to victory in the 1982 European Cup Final a 1–0 win against Bayern Munich. From 1975 to 1985 he made 406 appearances for Villa scoring 36 goals.
After leaving Villa, Dennis moved to Brighton & Hove Albion but was only there a year before returning to the Midlands with Birmingham City. During 1985-86 Dennis played 49 matches for the Seagulls.