Following graduation from Florida A&amp;M University, General Calhoun enlisted as a Private in the Florida Army National Guard in August 1977. After a six-year break in service, he received a direct commission and appointment to 1st Lieutenant in the Medical Service Corps on July 22, 1989. His follow-on assignments include Chief of Pharmacy Services, 131st Mobile Army Surgical Hospital (MASH); Executive Officer, 131st MASH; Logistics Officer, 50th Area Support Group (ASG); Director, Host Nations Support, 50th ASG and Director, Support Operations, 50th ASG. General Calhoun deployed with the 50th ASG in 2003 as Director of Host Nations Support during Operation Iraqi Freedom and served as Deputy Commander, Task Force (FL) in Mississippi during Hurricane Katrina.