Maclennan was first elected to Parliament in 1966 as Labour MP for Caithness and Sutherland, defeating the incumbent Liberal George Mackie. He served as a junior minister in the Labour government 1974-79. Following the Labour split in 1981, Maclennan became a founder member of the Social Democrats. In the wake of David Owen’s resignation, Maclennan was elected leader of the SDP in 1987 and led the party in the merger negotiations with the Liberal Party. Following the formation of the Liberal Democrats, Maclennan stood down as leader but remained as an MP until he retired in 2001 and became a life peer. He was President of the Liberal Democrats 1994-98 and was the party’s Cabinet Office spokesman in the House of Lords.