Mortimer was born in Liverpool. He began his career with Coventry City, where he came through the ranks and made over 200 appearances as a promising midfielder. His accomplished displays for Coventry attracted the attention of several top clubs, and he joined Aston Villa in 1975. Mortimer captained Villa to the 1982 European Cup Final. A 1–0 victory against Bayern Munich in the De Kuip Stadium was courtesy of a Peter Withe goal. From 1975 to 1985 he made 403 appearances for Villa scoring 36 goals.