 Graduating with a law degree, he was admitted to the Quebec bar in 1901 and was made King's Counsel in 1910.McMaster practiced law in Montreal with the firm of Fleet, Falconer, Cook, Brodie, Magee, Papineau, Campbell, Couture, Kerry and Bruneau before becoming Crown Attorney for the district of Montreal.Entering politics, McMaster was elected to the House of Commons of Canada in the 1917 federal election defeating Brigadier General Dennis Draper in the riding of Brome.