Gruenther was born in Platte Center, Nebraska, the son of Mary "Mayme" Shea, a school teacher, and Maximilian Gruenther, a newspaper editor who published the "Platte Center Signal". He attended St. Thomas Academy in Saint Paul, Minnesota. In June 1917, he entered the United States Military Academy at West Point and after studying for nineteen months, graduated early due to the war, on 1 November 1918, with a rank of fourth in a class of 277. He was commissioned a second lieutenant in the Field Artillery but after the Armistice he was recalled to West Point to complete his training, and graduated a second time in June 1919.