She grew up in Valley Cottage, New York. Leavey enlisted in the Marines in 2003 and was stationed at Camp Pendleton, California, where she was paired with military working dog Rex (E168). The pair served two deployments in Iraq together. They were first deployed to Fallujah in 2005, and then to Ramadi in 2006, where they were both wounded by an improvised explosive device. Leavey was awarded the Purple Heart and the Navy and Marine Corps Achievement Medal with a "V" device for heroism in combat.
Leavey first sought to adopt Rex after the 2006 bomb blast. Later, when Rex developed facial palsy that ended his bomb-sniffing duties, Leavey petitioned the Marine Corps for his adoption. They were reunited in April 2012 through the intervention of Senator Chuck Schumer. Rex died on December 22, 2012.
Leavey and Rex became the subject of the 2017 biographical film Megan Leavey. Leavey had a cameo appearance in the film as "Female Drill Instructor #3".
