He was signed by the Detroit Tigers in 1939 at the age of 18. A Tigers scout offered a $500 signing bonus to Newhouser, who found the sum of money to be unimaginable. Ten minutes after he signed, someone arrived from the Cleveland Indians; that employee had been prepared to offer $15,000 to his parents in addition to a $4,000 car.
Newhouser made his debut for Detroit on September 29, 1939. In 1940, he earned a spot on the Tigers out of spring training. In his first two full big-league seasons, the young left-hander experienced control problems, walking more batters than he struck out while posting win-loss records of 9–9 and 9–11. He improved in 1942 and 1943, posting excellent earned run averages (ERAs), but he still lost more than he won on a team with a weak offense.