Thybulle was born in Scottsdale, Arizona, the son of Greg, a Haitian-born engineer raised in Harlem, New York, and Dr. Elizabeth Thybulle, a naturopath who died of leukemia in 2015. When he was about two, his family moved to Sydney, Australia, where they lived for around seven years. Thybulle did not play much basketball in Australia. He was more interested in swimming there, where he said lifeguards "were a really big deal". His family returned to the U.S. in 2005 and settled in Sammamish, Washington, a suburb east of Seattle.
Thybulle could not make layups consistently until around the eighth grade, when his coordination began matching his speed. He attended Skyline High School for two years, then transferred to nearby Eastside Catholic, where he was ranked a four-star recruit by Scout.com and three-star by ESPN, and graduated in 2015.
