Upon his graduation from high school Colgan enlisted in the U.S. Army Air Corps and was called to active duty in February 1945. After graduation from basic training Colgan was stationed in Italy as part of a maintenance flight crew on a C-47. After completing his military service Colgan returned to Maryland where he met and married his wife Agnes. In 1948, Colgan and his wife moved to Washington, D.C. He trained as an airplane mechanic working for Capital Airlines and then obtained his commercial pilots license.