Robert Harcourt became the director of F. East. Harcourt and Company coal merchants. He was High Sheriff of Belfast in 1949, and later in the year unsuccessfully stood as the Ulster Unionist Party (UUP) candidate for South Down. Soon after this, John William Nixon, the independent Unionist Member of Parliament for Belfast Woodvale, died.
Harcourt stood in the ensuing by-election on 4 April 1950 and was elected.
In 1955, Harcourt was elected Lord Mayor of Belfast, a position which carried with it an ex officio position in the Senate of Northern Ireland. He served until 1957, when he was knighted.