In his first two years of high school, Mitchell played for Elizabeth Forward High School in Elizabeth, Pennsylvania, before transferring to Woodstock Academy in Woodstock, Connecticut. As a senior, he averaged 16.6 points, 13.5 rebounds, 3.6 assists and 2.6 blocks per game, leading his team to a 39–2 record and the National Prep Championship quarterfinals. Mitchell was named Connecticut Gatorade Player of the Year. He was selected as Power 5 Conference AAA Player of the Year and Hoophall Classic Most Valuable Player honors in both of his years at Woodstock. Mitchell competed for Expressions Elite on the Amateur Athletic Union circuit. He was a consensus four-star recruit and committed to play college basketball for UMass over offers from Georgia Tech, Indiana, Providence, Syracuse and Virginia Tech, among others.
