Coombe was president of the Barossa Political Reform League in the 1880s and an advocate of the Hare-Spence system of proportional representation. He stood as Liberal candidate for the seat of Barossa in the South Australian House of Assembly in 1896 then again in 1899, both times unsuccessfully, but won as a Labor candidate in 1901 when Sir John Downer vacated the seat to enter the Federal arena. He was reelected as an independent in 1902 and again in 1905. He was appointed Opposition whip in 1904 and Chairman of Committees in 1905. He was Commissioner of Crown Lands in the Peake Government, and chairman of the Royal Commission to enquire into the handling and marketing of wheat. He contested the 1906 election for the Liberal and Democratic Union. He held the seat until 1912, resigned from the Liberal and Democratic Union and joined Labor. He re-entered Parliament in 1915 for Labor, again as member for Barossa.