He worked for the Commercial Union insurance company for six years, however, one night following during an attendance at his girlfriend's local drama club rehearsals, Leonard became a member of the club. It was not long before his talents were in demand and Leonard was a member of no less than five drama societies. He also began elocution lessons to improve or lose his Liverpool accent. Having built up a decent reputation within the acting profession, Leonard left his job to become a full-time actor and by 1954 he performed as a professional actor in a stage play.

His professionalism and ability to learn and memorise huge volumes of dialogue landed him more roles in the following years. His reputation was becoming known up and down the country and following a few minor television appearances, Leonard landed more prominent roles such as Detective Inspector Bamber in the police drama, Z Cars. By 1964 Leonard was becoming more popular on television and had to sacrifice his preferred theatre opportunities. Supporting roles in Steptoe and Son and a Hollywood film appearance followed but his most acclaimed performances were at the West End, Saville Theatre in 1969 during which Leonard won three awards.

In 1974, Rising Damp, a new television comedy hit the TV screens and Leonard became a household name. Four series of the comedy were made and today survives as one of the greatest sitcoms of all time. The Fall and Rise of Reginald Perrin followed and by the 1980's his writing talents were also unleashed with the publication of several well received books.

In October 1984 at the Lyric Theatre on Shaftesbury Avenue, Leonard Rossiter collapsed and died of a heart attack aged 57.