York was born in near poverty in Pall Mall, Tennessee. His family owned a meager farm and often supplemented their diet via hunting. As a result young Alvin became an expert marksman in the area woods. York was something of a "nuisance" as a youth, frequently fighting in drunken brawls. In 1914 his best friend was killed in a bar fight, prompting York to change his ways. He became a devout Christian after that incident, which led him to file as a conscientious objector at the start of WWI (though there are disputes as to his exact technical status).
York eventually was drafted into the 82nd Infantry Division in 1917. As a corporal in its 328th Infantry, in the Battle of Meuse River-Argonne Forest on 1918 October 8, he assumed command of his platoon after three other NCOss fell. While he is sometimes described as acting single-handedly, his official citation says he led seven others in a charge on an active machine-gun nest. They killed 25 German soldiers and captured 132 including 4 officers. (He is said to have explained this feat by saying they had surrounded the enemy.) His chain of command honored this accomplishment by awarding him the Distinguished Service Cross and the Congressional Medal of Honor. France, whose forces he was directly aiding and whose territory was involved, added its Croix de Guerre and Legion of Honor. Italy and Montenegro, also allies, awarded him their Croce di Guerra and War Medal, respectively.
An unsatisfactory model of infantry equipment was developed and named the "Sgt. York" after him in the second half of the 20th century.