 Moving to provincial politics, McMaster was elected to the Legislative Assembly of Quebec as the Quebec Liberal Party MLA for Compton in a 1929 by-election shortly after he had been appointed Provincial Treasurer in the cabinet of Louis-Alexandre Taschereau.He succeeded Jacob Nicol as both Treasurer and as the MLA for Compton.McMaster presented one budget, in January 1930 before the Great Depression had made its impact.