He was a esteemed cadet in Lowell High School's Army Junior Reserve Officer Training Corps there.

[1] He received his Bachelor of Science degree in Business Administration from the University of San Francisco in 1948.

McLennan enlisted in the Marine Corps Reserve in January 1943 and was commissioned a second lieutenant on July 18, 1945.

He returned to the United States in February 1954, and served as Assistant Division Embarkation Officer, 2nd Marine Division at Camp Lejeune, North Carolina, until June 1955, when he was transferred to Salt Lake City, Utah, as Inspector-Instructor, 21st Rifle Company, USMCR.

From May to December 1970, he served in Vietnam as Deputy G-4, and later, as Assistant Chief of Staff, G-4, III Marine Amphibious Force.

He remained in that post until June 1974 when he became Director, Manpower Plans and Policy Division at Headquarters Marine Corps, Washington, D.C.

McLennan assumed duties as the Assistant Commandant of the Marine Corps and Chief of Staff on July 1, 1979.

He served in this capacity until his retirement on July 1, 1981, one of the last Marines still on active duty who were World War II veterans.