From 1913 until 1920 Colquhoun represented Mosman in the New South Wales Legislative Assembly as a Liberal-Nationalist. He was chairman of committees for his last five and a half months in parliament and was a strong debater and an authority on issues of constitutional law.
Later life
From 1912, Colquhoun was in legal partnership with George King and he retired in 1934. He served as a trustee of Taronga Zoo and in 1918 and 1919 he was the president of the Old Newingtonians' Union. Colquhoun died at home in Mosman and was survived by a son and a daughter.