Mobile Army Surgical Hospitals (MASH) were U.S. Army field hospital units conceptualized in 1946 as replacements for the obsolete World War II-era Auxiliary Surgical Group hospital units. MASH units were in operation from the Korean War to the Gulf War before being phased out in the early 2000s, in favor of combat support hospitals.

Each MASH unit had 60 beds, as well as surgical, nursing, and other enlisted and officer staff available at all times. MASH units filled a vital role in military medicine by providing support to army units upwards of 10,000 to 20,000 soldiers. These units had a low mortality rate compared to others, as the transportation time to hospitals was shorter, resulting in fewer patients dying within the "Golden Hour", the first hour after an injury is first sustained, which is referred to in trauma as the "most important hour". The U.S. Army deactivated the last MASH unit on February 16, 2006.

A precursor to MASH units, the American Expeditionary Force Mobile Hospital No. 1 was established, following the example of the French military automobile-chirurgical units shortly after the American entry to the war.

Principles for a mobile medical unit and their implementation were established through trial and error in the dental field during World War II by Major Vincent P. Marran, a medic in the United States Third Army. The effectiveness of his efforts were widely admired and supported by the command structure, but no formal designation was established. The first trials for what would become MASH units were established by the U.S. Army during World War II. The necessity for the U.S. Army to have more convenient treatment centers was shown by the long logistics of the stretched out supply lines during World War II. These units were known as "Auxiliary Surgical Groups" and would care for the wounded much closer than permanent hospitals, making them hospitals. In the early 1940s, Colonel Michael DeBakey and his colleague were selected to give recommendations on how to provide surgical care for the U.S. Army. The result was the ASG. Although these units were very inexperienced, they were incredibly effective resulting in five ASG units being created in 1943; this resulted in the ASG units being able to move along with the U.S. Army units located in Italy and was the basis in which MASH would directly come from.

Formally, the MASH unit was conceived by Michael E. DeBakey and other surgical consultants as the "mobile army surgical hospital". Col. Harry A. Ferguson, the executive officer of the Tokyo Army Hospital, also aided in the establishment of the MASH program. It was an alternative to the random individual systems of portable surgical hospitals, field hospitals, and general hospitals used during World War II. It was designed to get experienced personnel closer to the front, so that the wounded could be treated sooner and with greater success. Casualties were first treated at the point of injury through buddy aid, then routed through Battalion Aid Stations for emergency stabilizing surgery, and finally routed to the MASH for the most extensive treatment. This proved to be highly successful; during the Korean War, a seriously wounded soldier who made it to a MASH unit alive had a greater than 97% chance of survival once he received treatment. MASH units often took 24 hours to set up at new locations once moved with armored units, trucks and airmobile. Airmobile or the early stages, known today as helicopters, were crucial to the war effort as they were fast traveling units that could pickup casualties and deliver them effectively back to MASH units. With mountainous terrain in Korea, this was crucial as ground transport could compromise the patients lives and would take longer to arrive to MASH units.

In 1997, the last MASH unit in South Korea was deactivated. A deactivating ceremony was held in South Korea, which was attended by several members of the cast of the M*A*S*H television series, including Larry Linville (who played Frank Burns) and David Ogden Stiers (who played Charles Winchester).

The Vietnam War had little action for the MASH units within the U.S Army. The Vietnam War marked the demise of the MASH units as only one unit, known as the "2nd MASH Unit", served actively, and only from October 1966 to July 1967. The Vietnam War was a very different environment from the Korean War which MASH units were created in. The Vietnam War required a change from MASH to MUST, or "Medical Unit, Self-contained, Transportable" units. MUST units had trailers, inflatable sections, and technology focused on a wider range of wounds from war. In MUST units the wounds were greatly different than in the Korean War resulting in many changes needed to the units as a whole with new innovations required. New treatments were also needed for burn victims with MASH units suffering a mortality rate of up to 90% for burn victims. Although this clearly showed progress in the medical field, the type of warfare changed making MASH obsolete in many cases. MUST units had to keep their equipment on standby at all times effectively replacing MASH units and later transferring into more revolutionized units in war-zones. The idea of a MASH unit is to be available at all times to those who have been wounded in combat on the front and required general or trauma surgery but as technology got better, there was a lot more room to improve transportation, technology used to treat soldiers, and the layout of these units resulting in MASH units being converted to MUST units.

During the Gulf War, in September 1990, the main body of the 5th MASH, 44th Medical Brigade, XVIII Airborne Corps, Fort Bragg, North Carolina, deployed to King Abdul-Aziz Air Base, Royal Saudi Air Force, Daharan Saudi Arabia and was the first fully functional Army hospital in the country. Their forward surgical team and Advanced Party had deployed in mid August to Daharan. This unit moved forward six times, always as the first up hospital for the region. In February 1991, the 5th MASH was operationally attached to the 24th Infantry Division to provide forward surgical care (often right on the front battle lines) to the combat units that attacked the western flank of the Iraqi army. In March 1991, the 159th MASH of the Louisiana Army National Guard operated in Iraq in support of the 3rd Armored Division during Operation Desert Storm.