Born near Barrie, Upper Canada, Semlin worked there as a schoolteacher until 1862 when he moved to British Columbia during the gold rush to become a prospector.

He entered politics when British Columbia became a province of Canada, in 1871, winning the Yale riding in the provincial legislature in 1871 and was defeated in 1876, though won election again in 1882.

The site was chosen there as Cache Creek was the midpoint between the Cariboo region to the north and the populated areas of the Lower Mainland to the south.

His government lasted only two years and resigned to make way for the rump regime of Joseph Martin, who was defeated in the election of 1900.

Semlin raised a daughter, Mary, and left much of his estate, valued at just over $50,000 and consisting mainly of stock in the Dominion Ranch to her.