He was educated at Rongotai College and, from 1940 to 1941, Nelson College.
During World World War II he joined the Royal New Zealand Navy and served in the Royal Navy Fleet Air Arm as a Cadet (1943-1945). From 1950 to 1970 he was a civil engineering contractor and Director of R A McLennan Limited, McLennan"s Transport Limited, Flaxmere Quarries, Concrete Metal Limited, Amalgamated Contractors Limited, and Bulk Bitumen Limited.
From 1964 to 1970 he was National Secretary of the New Zealand Contractors Federation and from 1966 to 1971 the Scouts Association. In 1970 he was elected to the Nelson City Council, becoming Mayor from 1971 to 1980.
He was on the Nelson Harbour Board from 1971 to 1974.
During his term as Mayor a reclamation was proposed for residential purposes at Nelson Haven. There was strong public opposition and he decided at a public meeting that the proposal should not go ahead.
He did not have Council support at the time for his decision and the matter took some time to be finally resolved in the objectors" favour. In 2008, McLennan cut the cake with a sword in celebration of Nelson"s 150th anniversary of becoming a city.
He died at Nelson in 2013, and was buried in Marsden Valley Cemetery.
In the 1993 New Year Honours, McLennan was appointed a Member of the Order of the British Empire, for services to local body and community affairs