In 1917, Merritt graduated from The Citadel at the age of 19 and received a commission in the United States Marine Corps.

He and several other Citadel graduates became a part of the famous 'Devil Dogs' of Marine Corps legend at the Battle of Belleau Wood in 1918.

Merritt became one of the first Marine aviators qualified to fly off aircraft carriers and is credited with developing the concepts of dive bombing and close air support; his advocacy of air power was instrumental in helping integrate aviation into Marine combat doctrine.

During World War II, Merritt traveled to Cairo, Egypt to observe British air operations in desert warfare.

General Merritt was the only Marine Corps Aviator to serve in both the European and Asian Theaters of battle during World War II.