Mannix was elected as the Labor member for Liverpool at the 1952 by-election caused by the resignation of the former premier James McGirr who resigned from parliament to accept the chairmanship of the Maritime Services Board. He retained the seat for the next 6 elections and retired at the 1971 election.
Government
Mannix held ministerial positions in the governments of Robert Heffron and Jack Renshaw. He was initially appointed as an assistant minister in 1959 and held the Justice portfolio between 1960 and the defeat of the Labor government in 1965.