Born in Upper North Sydney, Cape Breton County, Nova Scotia, Maclean was educated at Pictou Academy and Dalhousie University.

He resigned as an MP in 1909, when he was re-elected to the House of Assembly and was appointed Attorney General and Commissioner of Crown Lands in the cabinet of George Henry Murray.

As a result of the Conscription Crisis of 1917 he crossed the floor on October 10, 1917, to support the Unionist government of Sir Robert Laird Borden and was a minister without portfolio in the Cabinet.

[1] His most significant decision is considered to be the one he wrote in R. v. Eastern Terminal Elevator Co.,[2] which was affirmed by the Supreme Court.

Maclean's decision is generally considered to be highly competent, where the issue was well thought out and analyzed, in contrast with Duff J's subsequent opinion at the SCC.