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 office as mayor of Monrovia from 1968 to 1976. He represented the California State Assembly in districts 61, 42, and 59 from 1978 to 1995. From 1982 to 1984 he served as chair of the Republican Caucus in the California Assembly.