He was born in small town of Rautajärvi near the border with the Russia, and started his military service in 1925. During the Winter War (1939-1940) between Finland and Russia, he began his duty as a sniper against the Red Army. Working in temperatures between -20 to -40 Celsius, and dressed completely in a white camouflage suit, Häyhä killed 505 Red Army soldiers. He used a Mosin-Nagant M28 rifle because it suited his small frame (5 ft). He preferred to use iron sights rather than telescopic sights to present less of a target (sun reflecting from lenses gave the position away, and you have to raise your head higher with telescopic sights).
On March 6, 1940, Häyhä was shot in the jaw and went into a coma. He regained consciousness on March 13, the day that peace was declared. Shortly after the war, Häyhä was promoted straight from a sergeant to an ensign by Field Marshal Mannerheim. When asked in 1998, aged 93, how he had become such a good shot, he answered, "Practice."