Roger Hugh Charles Donlon was born on January 30, 1934 in Saugerties, New York. He enlisted in the United States Air Force in 1953, and was appointed to attend the United States Military Academy, West Point in 1955. He resigned from the Academy for personal reasons, but later reenlisted in the United States Army in 1958.
Donlon attended Officer Candidates School and was commissioned as an officer. In August 1963 he joined the United States Army Special Forces.