Lieutenant Gary Mullen was born and raised in Cincinnati, Ohio. He attended St. Xavier High School where he was a junior varsity wrestler. He graduated with Second Honors in 1990. Gary continued his education at Miami University, Oxford, Ohio. While there, he became a PADI open water SCUBA instructor and made frequent trips to northern Florida to teach diving. He graduated May 1994 with a Bachelor of Arts degree in chemistry. He then attended University of Cincinnati College of Medicine, graduating with a Doctor of Medicine degree in June 1998. After completion of medical school, Gary completed his internship in obstetrics and gynecology East Tennessee State University in Johnson City, Tennessee. In January 1999, during his internship year he was commissioned as a lieutenant.

In September 1999, Gary reported to the Naval Operational Medical Institute in Pensacola, Florida, for flight surgery training. Gary received his naval flight surgeon wings in April 2000. In May 2000, Gary reported to the Fleet Composite Squadron 8 (VC 8) "Redtails," forward deployed in Ceiba, Puerto Rico. He participated in numerous emergent medical evacuations and search and rescue missions in support of the U.S. Atlantic Fleet, foreign fleets, and civilian humanitarian missions.

In May 2002, Gary reported to as flight surgeon for Training Air Wing 2 (CTW 2) in Kingsville, Texas. He provided support for training squadrons VT 21 and VT 22 for intermediate and advanced jet flight training.

Gary joined the Blue Angels in September 2002. His personal decorations include the Navy Achievement Medal and various other service and unit awards.