Marshall was born in Catskill, New York and raised in Colorado, California and El Paso, Texas. He joined the Army in 1917 and saw service on the border with Mexico during the Pancho Villa Expedition before serving in France during World War I. He attained the rank of sergeant while serving as a member of Company A, 315th Engineer Regiment, 90th Infantry Division. The 315th Engineers participated in the Saint-Mihiel and Meuse-Argonne Offensives.
Shortly after Saint-Mihiel, Marshall was one of the soldiers in his unit selected to take the entrance examinations for the United States Military Academy as part of an Army initiative to replenish the officer corps with exceptional soldiers from the ranks. (Under this program, Captain Harry S. Truman nominated a member of his battery, John Francis Uncles. Uncles retired as a lieutenant general.) Marshall subsequently attended Officer Candidate School, received his commission in early 1919, and remained in France to assist with post-war demobilization.
A 1989 article by historian Frederic Smoler questioned Marshall's research methods as a historian, indicating that Marshall had exaggerated and inflated his World War I experiences to give himself a reputation for having led soldiers in combat, which would enhance his credibility as a historian. Smoler contended that the 315th Engineers were a rear-echelon unit, and that Marshall did not participate in combat during the war.
Subsequent investigation by Marshall's grandson, John Douglas Marshall, included in his book Reconciliation Road: A Family Odyssey of War and Honor details S. L. A. Marshall's contemporary letters to his father. These letters indicate that Marshall took part in both Saint-Mihiel and Meuse-Argonne, and was "slightly" gassed at Saint-Mihiel. In addition, John Douglas Marshall's book recounts S. L. A. Marshall's inscription inside the front cover of his World War I scrapbook, which he dedicated to a fellow 315th Engineers soldier who was killed in action on November 8, 1918. According to the inscription, the soldier was shot by Germans while the 315th Engineers were taking part in action near Bantheville during the final days of the Meuse-Argonne offensive, and Marshall was with him when it happened. John Marshall's subsequent investigation revealed that the friend was hit by artillery fire, not shot, and that S. L. A. Marshall was taking the West Point entrance exams that day. John Marshall ultimately concluded that, while his grandfather exaggerated some claims about his wartime experiences, many are valid, and that the body of his grandfather's later work still has value.
After Marshall's discharge at the end of the war, he remained in the Reserve, attended the Texas College of Mines (now the University of Texas at El Paso), and worked at a variety of jobs, including bricklayer. In the early 1920s, he became a newspaper reporter and editor, first with the El Paso Herald, and later The Detroit News. As a reporter, Marshall gained a national reputation for his coverage of Latin American and European military affairs, including the Spanish Civil War.
In 1940, Marshall began a career as an author with the publication of Blitzkrieg: Armies on Wheels, an analysis of the tactics the Wehrmacht developed in the years leading up to the start of World War II.
During World War II, Marshall became an official Army combat historian, and came to know many of the war's best-known Allied commanders, including George S. Patton and Omar N. Bradley. He conducted hundreds of interviews of both enlisted men and officers regarding their combat experiences, and was an early proponent of oral history techniques. In particular, Marshall favored the group interview, where he would gather surviving members of a frontline unit together and debrief them on their combat experiences of a day or two before.
Marshall's work on infantry combat effectiveness in World War II, titled Men Against Fire, is his best-known and most controversial work. In the book, Marshall claimed that of the World War II U.S. troops in actual combat, 75% never fired at the enemy for the purpose of killing, even though they were engaged in combat and under direct threat. Marshall argued that the Army should devote significant training resources to increasing the percentage of soldiers willing to engage the enemy with direct fire.
Less well known, but perhaps more significant was Marshall's effort to assemble German officers after the war to write histories and analyses of battles in all theatres of the European war. At the height of the project, over 200 German officers participated, including Heinz Guderian and Franz Halder. Hundreds of monographs came out of the project, of which three are available in commercial print.
Marshall was recalled in late 1950 for three months' duty as a Historian/Operations Analyst for the Eighth Army during the Korean War. He collected his numerous Korean combat interviews into a treatise analyzing U.S. infantry and weapons effectiveness, Commentary on Infantry and Weapons in Korea 1950–51. The U.S. Army decided to classify some of Marshall's findings as restricted information, later incorporating them as part of a plan to improve combat training, weapons, equipment, and tactics.
Following his retirement from the Army Reserve in 1960 with the rank of brigadier general, Marshall continued to serve as an unofficial adviser to the Army. As a private citizen, he spent late 1966 and early 1967 in Vietnam on an Army-sponsored tour for the official purpose of teaching his after-action interview techniques to field commanders, in order to improve data collection for both the chain of command and the future official history of the Vietnam War.
The Army Chief of Military History's representative on the tour, Colonel David H. Hackworth, collected his own observations from the trip and published them as The Vietnam Primer, giving Marshall credit as co-author.
Marshall died in 1977 in El Paso, Texas, and was buried at Fort Bliss National Cemetery, Section A, Grave 124.
Some veterans and historians have cast doubt on Marshall's research methodology. Professor Roger J. Spiller (Deputy Director of the Combat Studies Institute, US Army Command and General Staff College) argues in his 1988 article, "S.L.A. Marshall and the Ratio of Fire" (RUSI Journal, Winter 1988, pages 63–71), that Marshall had not actually conducted the research upon which he based his ratio-of-fire theory. "The 'systematic collection of data' appears to have been an invention." This revelation has called into question the authenticity of some of Marshall's other books and has lent academic weight to doubts about his integrity that had been raised in military circles even decades earlier.
In his 1989 memoir, About Face, Hackworth described his initial elation at an assignment with a man he idolized, and how that elation turned to disillusion after seeing Marshall's character and methods firsthand. Hackworth described Marshall as a "voyeur warrior," for whom "the truth never got in the way of a good story", and went so far as to say, "Veterans of many of the actions he 'documented' in his books have complained bitterly over the years of his inaccuracy or blatant bias".
The University of Texas at El Paso library has a special collection built around his books.
Marshall appears as a character in Brothers in Arms: Earned in Blood, a video game released in 2005.
The series 3 Black Mirror episode, "Men Against Fire" (2016), was partly inspired by Men Against Fire: The Problem of Battle Command and explores the same themes.