Mason was elected Mayor of Pukekohe in 1915. He was left-wing in his political outlook, and joined the Labour Party on its foundation in 1916. In the 1919 general election, he was Labour's candidate for the seat of Manukau, but was defeated. Mason sought the Labour nomination for the in the seat, but was beaten by John A. Lee. Later, he shifted his attention to the seat of Eden — he contested it in the 1922 election and 1925 election. 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.