Munro was born on October 11, 1919, in Vancouver, British Columbia, to James Munro, originally from California, and Edith Thrower Fairey from Liverpool, England. The family moved to Vancouver, Washington, where his father worked as an electrician for Warren Construction Company. He became a naturalized U.S. citizen in 1922 along with his sister and mother and grew up in South Cle Elum, Washington. He was educated at South Cle Elum Grade School and graduated from Cle Elum High School in 1937. He attended Central Washington College of Education (now known as Central Washington University) for a year before leaving to enlist in the United States Coast Guard in 1939. He had an outstanding record as an enlisted man and was promoted rapidly through the ratings to a signalman, first class.