Munn was born in Winnipeg, Manitoba,[2] and was a grandson of Stuart Jenkins, a well-known writer in the 1890s for Scientific American.

Upon graduation in 1941, Munn became a weather forecaster, spending 1943 to 1949 in Gander, Newfoundland where he briefed RAF Ferry Command and other wartime pilots on their way to Britain.

He served as president of the Canadian Branch of the Royal Meteorological Society, 1964–66, and on several international committees, including WMO and ICSU[clarification needed].

He spent a winter in Stockholm working with Bert Bolin on the acid rain issue, and for 17 years he was editor-in-chief of the SCOPE series of 60 books relating to interdisciplinary environmental topics.

[9] Munn was elected a Fellow of the Royal Society of Canada in 1985[10] and published several books and many scientific articles on environmental topics.