Stewart was born May 6, 1917, in Pilot Mound, Manitoba, where he learned to play hockey on the community's outdoor rinks. He had three sisters. In the off-seasons, he returned to his family farm, work which was said to have given him the strength he displayed in the NHL. An avid sportsman, Stewart was an active curler during his playing career, and played softball in the summer. Though he was regarded on the ice as one of toughest players in the game, he was also quiet. He was known as "Silent Jack" by his teammates, allowing his actions to speak for him.