He was a member of the South Australian House of Assembly from 1875 to 1878 and 1881 to 1911, representing the electorates of Light (1875–78, 1881–1902) and Wooroora (1902–1911).

Coles obtained a position as a junior clerk with the Murray River Navigation office, but gave this up to become assistant dispenser and receiver of stores at the Adelaide hospital for three years.

On leaving this service he became an auctioneer and stock salesman and a member of the real estate firm of Coles and Goodchild, with which James White was later associated.

He was returned to the South Australian House of Assembly as member for Light in 1875, but did not stand at the 1878 election as he found that the strain of carrying on both business and parliamentary duties was too great.

In 1890 he was elected speaker of the house of assembly in succession to Sir John Bray, and held the position until he resigned, about three weeks before his death of Bright's disease on 6 December 1911.