He served in the Legislative Assembly of Ontario from 1981 to 1987, as a member of the Progressive Conservative Party.
[3] For the next four years, he served as a backbench supporter of the Bill Davis and Frank Miller administrations.
The Progressive Conservatives were reduced to a tenuous minority government under Miller's leadership in the 1985 provincial election and Treleaven was re-elected in Oxford.
[5] In 1986, he was a prominent opponent of Peterson's plan to ban extra billing by Ontario doctors.
Treleaven was defeated in the 1987 provincial election, losing to Liberal candidate Charlie Tatham by 2,874 votes.