Before the start of his Junior year, Spellman decided to attend the MacDuffie School in Granby, Massachusetts.

Despite not being named a McDonald's All-American, Spellman was selected to play in the 2016 Jordan Brand Classic where He had a double-double of 12 points and 15 rebounds.

[citation needed] Spellman was rated as a five-star recruit and considered a top-15 player of the 2016 high school class.

[5] Spellman was forced to redshirt his freshman year as a partial academic qualifier since he did not graduate with his original high school class.

[11][12] On July 8, 2019, Spellman was traded to the Golden State Warriors for Damian Jones and a 2026 second round draft pick.

[13] On February 6, 2020, Spellman was traded to the Minnesota Timberwolves, along with D'Angelo Russell and Jacob Evans, in exchange for Andrew Wiggins and a first round pick.

[15] He recorded a triple-double with the Wolves on March 8, posting 18 points, 14 rebounds, and 12 assists in a 115–112 loss to the Santa Cruz Warriors.

[21] In March 2023, Anyang KGC played in the EASL Champions Week, and won the gold medal.

[27] On February 10, 2025, Spellman signed with Wonju DB Promy of the Korean Basketball League (KBL), replacing Robert Carter.

[28][29] On April 18, 2023, Spellman acquired Lebanese citizenship, and joined the Lebanon men's national basketball team.