Mountjoy served as the mayor of Monrovia from 1968 to 1976. He served in the California State Assembly for the 61st, 42nd, and 59th district from 1978 to 1995. From 1982 to 1984 he served as the California Assembly Republican Caucus Chair.
Mountjoy held municipal office as mayor of Monrovia from 1968 to 1976. He later served in the state legislative body, representing districts 61, 42, and 59 in the California State Assembly from 1978 to 1995. From 1982 to 1984 he headed the Republican faction within the California Assembly.