The United States Maritime Service (USMS) was established in 1938 under the provisions of the Merchant Marine Act of 1936 as voluntary training organization to train individuals to become officers and crewmembers on merchant ships that form the United States Merchant Marine per 46 U.S.C. § 51701. Heavily utilized during World War II, the USMS was largely dissolved in 1954, and its resources were absorbed into other federal departments. However, while the service is no longer structurally organized, remnants of the service still exist today and the service still actively commissions officers to function as administrators and instructors at the United States Merchant Marine Academy and the state maritime academies.

The United States Merchant Marine is an organization composed of United States civilian mariners and U.S. civilian and federally owned merchant vessels. Both the civilian mariners and the merchant vessels are managed by a combination of the government and private sectors, and engage in commerce or transportation of goods and services in and out of the navigable waters of the United States. The Merchant Marine primarily transports domestic and international cargo and passengers during peacetime, and operate and maintain deep-sea merchant ships, tugboats, towboats, ferries, dredges, excursion vessels, charter boats and other waterborne craft on the oceans, the Great Lakes, rivers, canals, harbors, and other waterways. In times of war, the Merchant Marine can be an auxiliary to the United States Navy, and can be called upon to deliver military personnel and materiel for the military.