Maclean practised as a solicitor before becoming a Liberal Member of Parliament. He represented a number of constituencies, first Bath (1906-Jan 1910), next Peebles and Selkirk (Dec 1910-1918), Peebles and South Midlothian (1918-1922), and finally the Northern Division of Cornwall from 1929 to 1932.
He was knighted and became a Privy Councillor in 1916, and was Leader of the Liberal Parliamentary Party from 1918 to 1922, as the nominal leader of the Liberal Party, Herbert Henry Asquith had lost his seat in the House of Commons. For two years he also served as Leader of the Opposition, while Labour had no official leader and Sinn Féin refused to participate in parliamentary government.
Towards the end of his life, Maclean joined the National Government, a coalition. He served as President of the Board of Education from 1931 to 1932, when he died from cardiovascular disease at the age of sixty-eight.
Maclean is buried with his wife, Gwendoline Katherina Leonora Hope Maclean, born on 30 March 1878 and died 28 March 1970, great-great-great-granddaughter of the 1st Earls of Darlington, whom he married on 26 October 1907, and his eldest son, Ian, in the churchyard of Holy Trinity Church, Penn, Buckinghamshire. He was the father of Sir Hector Charles Donald Maclean, married in 1933 to Opre Vyvyan, born on 27 September 1910, descended from the baronets Vyvyan of Trelowarren and the German Schmiedern barons, maternal grandparents of Rupert Everett. Another of his four sons was the diplomat and spy, Donald Duart Maclean.