Roderick Diarmid Maclennan was born on 13 October 1898 at Laggan Inverness-shire to Duncan Macrae Maclennan and Isabella Margaret Macpherson. His early education took place in New Zealand from 1903 to 1912 before he returned to Scotland to study at Kingussie (1914–15), Oban High School (1916–17) and the University of Edinburgh. Maclennan began his MA in philosophy in 1917, but his studies were interrupted by military service in the First World War. He was wounded while serving in France in 1918. He completed his MA with first class honours in 1925.
After studying divinity at Edinburgh from 1925 to 1927 during which time he was licensed to preach by the Presbytery of Lochcarron in 1926 Maclennan was appointed a lecturer in philosophy. He served as professor of philosophy and chair of the department at the University of McGill (Montreal) from 1933 to 1956 upon which time he returned to Scotland to be ordained and inducted into the charge of Kintail Lochcarron. He retired from the charge on 31 May 1964.
Maclennan married Mary Pierce Colby in 1936 with whom he adopted Mary Janet (b. 3 July 1949). He was made a member of both the Scottish Philosophical Society and the American Philosophical Society. Edinburgh University conferred the degree of DD to him in 1962. Although he delivered the Gifford Lectures in 1959–1960 they were never published. Maclennan’s sole published piece of work was the volume of the collected papers of Norman Kemp Smith he co-edited entitled The Credibility of Divine Existence (Macmillan, 1967). Maclennan died in 1977.