CDR Gillen was born in Lexington, KY and grew up in Queens, NY. He attended the University of Wisconsin at Madison, graduating in 1992 with a Bachelor of Arts in Political Science. Upon graduation, he received a commission in the Navy through the Naval Reserve Officer Training Corps.
As a Surface Warfare Officer, his first sea assignment was as the Main Engine Division Officer on USS MOBILE (LKA 115). In his second division officer tour, he was on the commissioning crew of USS RUSSELL (DDG 59), serving as the Electrical Officer and later Ordnance Officer. For his department head tours, he was the Chief Engineer on USS DOYLE (FFG 39) and USS CAPE ST. GEORGE (CG 71). In his last sea tour, he served as the Executive Officer on the USS SAN JACINTO (CG 56).
For shore duty, he attended the Naval Postgraduate School in Monterrey, graduating with a Masters in Management, subspecialty in Financial Management. He served in the Pentagon on the OPNAV Staff, Surface Warfare Division (N76), as a Littoral Combat Ship requirements officer and later an aide to the Director, Surface Warfare Division. After his executive officer tour, he was assigned as a Surface Warfare placement officer and an O-4/O-5 detailer at the Bureau of Naval Personnel (PERS 41). CDR Gillen attended the Air War College in Montgomery, AL and received an additional Masters in Strategic Studies.