Toussaint was born in Pembroke Pines, Florida, to Dany Toussaint and Kahaso Kiti. He is of Haitian and Kenyan descent. Toussaint's nickname, Touki, is a portmanteau of his parents' surnames. The family moved to Haiti when he was three months old. Toussaint and his mother moved back to Florida when he was six. He stopped playing baseball at age 10 due to a strikeout-filled season as a batter, and focused on soccer and hockey only to return to baseball two years later. Toussaint attended Coral Springs Charter School for his freshman year before transferring to Coral Springs Christian Academy also in Coral Springs, Florida. As a junior in 2013, Toussaint was a MaxPreps All-American after he was 6–2 with a 0.78 earned run average (ERA) and 83 strikeouts. He also hit .458 with six home runs and 32 runs batted in. He was named Broward County's Class 5A-4A-3A-2A Player of the Year. Toussaint committed to play college baseball at Vanderbilt University.