 MacArthur contested Greenock twice as a Unionist in 1955, in the general election and a by-election.He was Member of Parliament for Perth and East Perthshire from 1959 until his defeat at the October 1974 general election, when he lost by 793 votes to Douglas Crawford of the Scottish National Party.In the House of Commons he was a whip 1963–65, as a Lord Commissioner of the Treasury 1963–64.