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 commission officers to function as administrators and instructors at the United States Merchant Marine Academy and the state maritime academies.

The U.S. Maritime Service falls under the authority of the Maritime Administration of the U.S. Department of Transportation. The Commandant of the United States Maritime Service is the Maritime Administrator, who also serves as the director of the National Shipping Authority and the chairperson of the Maritime Subsidy Board. The Secretary of Transportation may determine the number of individuals in the service, set the rates of pay, prescribe the service's uniform, and create and issue awards for the service. By law, the U.S. Maritime Service's rank structure must be the same as that of the U.S. Coast Guard, but uniforms worn are those of the U.S. Navy with distinctive USMS insignia and devices. The U.S. Maritime Service is not part of the structurally organized uniformed services of the United States. The officers in the service cannot claim veteran status or military benefits including retirement (unless earned separately by previous, concurrent, or subsequent military service). Uniformed officers serving in the U.S. Maritime Service cannot be deployed or assigned to another military service, and thus are not subjected to the Uniform Code of Military Justice.

Graduates of the state maritime academies Student Incentive Payment Program and graduates of the U.S. Merchant Marine Academy are required to report annually to the Maritime Administration to comply with their service obligations.

Superintendents or presidents of the seven maritime academies in the U.S. are frequently commissioned in the U.S. Maritime Service, such as the superintendent of the United States Merchant Marine Academy. Additionally, some administrators and instructors at the maritime academies may be assigned rank in the Maritime Service. Those with U.S. Maritime Service rank indicate that by listing their rank and name, followed by "USMS".

The merchant marine in the United States was in a state of decline in the mid-1930s. At that time, few ships were being built, existing ships were old and inefficient, maritime unions were at war with one another, ship owners were at odds with the unions, and the crews' efficiency and morale were at an ebb. Congress took action to fix the problems in 1936. The Merchant Marine Act, approved on 29 June 1936, created the U.S. Maritime Commission "to further the development and maintenance of an adequate and well balanced American merchant marine, to promote the commerce of the United States, and to aid in the national defense."

The commission realized that a trained merchant marine work force was vital to the national interest. At the request of Congress, the chairman of the Maritime Commission, VADM Emory S. Land worked with ADM Russell R. Waesche, Commandant of the Coast Guard, to formulate a training program for merchant-marine personnel. Called the U.S. Maritime Service, the new training program was inaugurated in 1938. It used a combination of civilian Maritime Commission and uniformed Coast Guard instructors to advance the professional training of merchant mariners.

As with the other military services, the entry of the United States into the Second World War necessitated the immediate growth of the merchant marine and the Coast Guard. The Maritime Commission spawned the War Shipping Administration in early February 1942. This new agency received a number of functions considered vital to the war effort, including maritime training. Several weeks after the creation of the new agency, however, the Maritime Service was transferred again to the Coast Guard (on 28 February of that year, under Executive Order 9083; the marine safety aspects of the Bureau of Marine Inspection and Navigation (BuMIN) were also transferred to the Coast Guard at this time). The transfer allowed the War Shipping Administration to concentrate on organizing American merchant shipping, building new ships, and carrying cargoes where they were needed most.

The Maritime Service was later transferred to another agency, while marine inspection and licensing continued to be Coast Guard missions. The need for administering the merchant marine during wartime was demonstrated during the First World War. Commerce warfare, carried on by submarines and merchant raiders, had a disastrous effect on the Allied merchant fleet. With the resumption of unrestricted submarine warfare in 1917, U-boats sank ships faster than replacements could be built. The United States intended to meet this crisis with large numbers of mass-produced freighters and transports. When World War II loomed, the Maritime Commission began a crash shipbuilding program utilizing every available resource. The experienced shipyards built complicated vessels, such as warships. New shipyards, which opened almost overnight around the country, generally built less sophisticated ships such as the emergency construction Liberty ships. By 1945 the shipyards had completed more than 2,700 "Liberty" ships and hundreds of Victory ships, tankers and transports.