 He finally won Eden in a 1926 by-election, assisted by the fact that the Reform Party's vote was split by a defeated nominee, Ellen Melville.Rex Mason represented the seat of Eden in the 22nd parliament (1926–28), Auckland Suburbs in the 23rd to 27th parliaments (1928–46), Waitakere in the 28th to 33rd parliaments (1946–63), and New Lynn in the 34th parliament (1963–66).In both 1931 and 1933 he stood unsuccessfully for Mayor of Auckland City on a Labour Party ticket, defeated by George Hutchison on both occasions.