It was named for Captain Robert M. Losey, an aeronautical meteorologist who in April 1940 became the United States' first military casualty in World War II.

Units assigned were: Subsequently: Since the Navy left Fort Allen in 1980 there's been no regular Active Duty military personnel assigned on post, but the Puerto Rico Army National Guard and U.S. Army Reserve do have permanent Active Guard Reserve personnel present.

LTC Hernan G. Pesquera United States Army Reserve Center (USARC) is located on 16 hectares (40 acres) within the grounds of Fort Allen, Puerto Rico.

The Puerto Rico Army National Guard 201st Regiment Multifunctional Training Brigade was located on the Fort Allen Armed Forces Reserve Center.

The United States Navy leases 47-hectare (117-acre) of the other than operational area for a Relocatable Over the Horizon (ROTHR) receiver site was built in 1997 on government land at Fort Allen as part of a surveillance network which is designed to monitor flights over an area encompassing more than 1,000,000 square miles (2,600,000 km2) in South America.