McGonagle was born November 19, 1925 in Wichita, Kansas. After attending secondary school and college in California, he enlisted in the United States Navy in 1944 and for the next three years participated in a navy training program at the University of Southern California. In June 1947 he accepted a commission in the navy as an ensign. He was assigned to the destroyer and after that was posted to the minesweeper from 1947–1950. During the Korean War he served on the minesweeper during the extensive operations that earned him and the other members of the crew a Presidential Unit Citation. From 1951 to 1966, he was assigned to various positions ashore and afloat, including commands of the fleet tug from 1957 to 1958 and the salvage ship from 1961 to 1963.