Mullinnix was killed in action while commanding a Navy escort carrier division and task force group when his flagship, the escort carrier USS Liscome Bay (CVE-56), was sunk by a Japanese submarine near the Gilbert Islands during World War II.

He was the fourth of five US Navy admirals killed in battle during WWII, including: Isaac C. Kidd (1941, Attack on Pearl Harbor); Norman Scott and Daniel J. Callaghan (same day, 1942, Naval Battle of Guadalcanal); and Theodore E. Chandler (1945, invasion of Lingayen Gulf).

[1] Declared dead a year later, he was posthumously awarded the Legion of Merit: For exceptional meritorious conduct...as Commander of a Carrier Air Support Group prior to and during the amphibious invasion of Japanese-held Makin Atoll, Gilbert Islands from November 1 to 24, 1943.

Displaying outstanding initiative and superior executive ability, Rear Admiral Mullinnix skillfully conducted anti-submarine and air combat patrols supporting our landing operations on this strongly defended island and, through his brilliant leadership, enabled escort carriers to carry out a well coordinated, aggressive attack against the enemy.

Rear Admiral Mullinnix's tireless efforts, meticulous attention to detail and loyal devotion to the accomplishment of an extremely difficult and hazardous mission contributed materially to our subsequent capture of this strategic area.