From 1870 to 1885 and again from 1893 to 1906 he was a member of the colony's appointed Legislative Council, the Upper House of Newfoundland's parliament.

Thorburn was an opponent of Sir William Whiteway's plans to build a cross-Newfoundland railway as a means of diversifying and industrialising the economy.

Thorburn, a leading merchant, argued that the colony should be developed along strict commercial lines based on the fisheries.

An economic downturn was exacerbated by the colony's one industry economy forcing Thorburn to belatedly reverse himself and implement a public works agenda.

It was too late, however, and Thorburn was defeated in 1889 by Whiteway and his new Liberal Party which had been created to promote the railway plan.