Stewart was born in 1979, in Toronto, Ontario, the son of Sandra and Brian Stewart. He began underwater photography as a teenager, and became a scuba diving trainer at eighteen years old. He attended Lawrence Park Collegiate Institute and Crescent School in Toronto as a youth.
For four years, Stewart was chief photographer for the Canadian Wildlife Federation's magazines, and a freelance journalist. He won awards for his journalism. He held a bachelor's degree in biology from the University of Western Ontario, and studied zoology and marine biology in Kenya and Jamaica.