He played basketball for Pine Crest School in Fort Lauderdale, Florida, where he was rated a five-star recruit by ESPN and won a state title.

[7] He averaged 20.5 points, 8.4 rebounds and 4.4 assists per game as a junior, leading his team to the Class 4A state championship in a 90–83 double overtime win over Santa Fe High School.

[8] In the title game, Thompson scored 43 points and helped Pine Crest overcome an eight-point deficit with 45 seconds left in overtime.

[12] Sources: On May 25, 2021, Thompson signed a two-year contract with Overtime Elite (OTE), a new professional league based in Atlanta with players between ages 16 and 20.

He joined the league with his brother Ausar, bypassing his final year of high school and college, because he felt that it would prepare him best for the NBA.

[19] In the 2022–23 OTE season, Thompson played for the City Reapers alongside team captain Ausar, who selected him with the first pick in the league's draft.

[20] On January 9, 2023, he was named OTE Player of the Week, three days after recording 22 points, 10 rebounds, seven assists and five steals in a 101–90 win over the Cold Hearts.

[30] Thompson made his return on the bench after a 15-game absence and recorded two points, five rebounds, and one assist in 10 minutes in the 93–82 win over the San Antonio Spurs.

Thompson's minutes and numbers increased while playing full-time in the starting lineup for the remaining 15 games after teammate Alperen Şengün missed the remainder of the season from a knee injury.