Thornwell was born Nov. 15, 1994, the son of Greg Wade and Sharicka Thornwell. He has one brother, Quantavious, and one sister, Quasheka. For his first three years of high school, Thornwell attended Lancaster High School in Lancaster, South Carolina. For his senior year, he transferred to Oak Hill Academy, where he averaged 26.8 points, 8.6 rebounds and 3.6 per game. The team had a 33-5 record, and Thornwell was ranked as the 39th-best player by ESPN.com and a four-star recruit by Rivals.com. He also played in the 2012 Nike Global Challenge. He attended South Carolina for college, where he had a standout senior season, averaging 21.4 points, 7.1 rebounds, 2.8 assists, 2.1 steals and 1.0 blocks. That led him to being named SEC Player of the Year.