DeSantis was commissioned as an officer in the U.S. Navy Judge Advocate General's Corps (JAG).

He completed Naval Justice School and served as a prosecutor at Naval Station Mayport, Florida.

DeSantis was stationed at Joint Task Force Guantanamo, where he worked with detainees at the Guantanamo Bay detention camp.

He deployed to Iraq as a legal adviser to SEAL Team One, serving in Fallujah.

DeSantis was honorably discharged from active duty and later served in the Navy Reserve until 2019.