DeSantis volunteered and joined the U.S. Navy and was assigned to the Judge Advocates Generals Corps for the Naval Station Mayport and also worked for the Joint Task Force-Guantanamo, Guantanamo Bay Joint Detention Facility, and Naval Special Warfare Command as a military lieutenant. He was deployed to the Iraq War along with other colleague troops in 2007 and later returned to the United States in 2008. DeSantis served as a naval and military officer from 2004 until his honorable discharge in February 2010.