He joined the New Zealand Labour Party in 1932, but when John A. Lee was expelled from the party in 1940, Douglas followed to join the new Democratic Labour Party. He rejoined the Labour Party in 1952 and represented the Auckland Central electorate in Parliament from 1960 until his retirement in 1975, serving time on the Opposition front bench. He lost his left arm in a duck-shooting accident in 1927.
He became president of the branch in 1935.
That same year he was elected to the Auckland City Council for Labour and served three years until Labour"s defeat. He became the assistant secretary of the Auckland Coach and Carolina Builders" Union and the Auckland Brewers", Wine and Spirit Merchants" Employees" Union in 1936, and then secretary of both unions the following year, remaining in that post for the latter union until 1963.
He was secretary of the Auckland Trades Council from 1939 to 1941 and led the Labour Party"s Junior Labour League. When Lee was expelled from the Labour Party in 1940, Douglas left also and helped him set up the Democratic Labour Party.
He ran for Parliament in 1941 (1941 by-election in Waitemata)) and in the 1943 election (for Onehunga) but was defeated.
He operated a bookselling business for about 15 years from 1944, first with Lee and then on his own after he and Lee fell out in 1954. Douglas rejoined the Labour Party in 1952. When his father-in-law Bill Anderton, Labour Member of Parliament for Auckland Central, retired from Parliament in 1960, Douglas was elected in his place.
He served as president of the Labour Party from 1966 to 1970, and sat on the Opposition front bench as spokesperson for education, social security and industrial relations from 1967 to 1972.
When Labour came to power in 1972, Douglas missed selection for cabinet and took himself to the back benches in disappointment. He retired from Parliament at the 1975 general election.
Douglas married Dorothy Jennie Anderton, a daughter of fellow politician Bill Anderton, in 1937. Douglas was awarded a QSO in 1976.
He died in Auckland in 1985.
Joining the Grey Lynn branch of the Labour Party in 1932, he became a close friend of (Member of Parliament) John A. Lee (who lost his left arm in World War I). He was a member of the party"s national executive and edited John A. Lee"s Weekly.