Thornwell was selected by the Milwaukee Bucks with the 48th overall pick in the 2017 NBA draft before he was traded to the Los Angeles Clippers.

As a result of his production that season, he was not only a member of the All-SEC First Team, but he was also named the SEC Player of the Year thanks to votes from the coaches of the Southeastern Conference.

[4] On August 4, 2019, Thornwell signed with the Cleveland Cavaliers and played in three preseason games before he was waived on October 16.

On October 26, 2019, the Canton Charge traded Thornwell to the Rio Grande Valley Vipers for a first-round G-League pick in 2020.

[14] On October 19, 2022, Thornwell has signed with Frutti Extra Bursaspor of the Turkish Basketbol Süper Ligi (BSL).

[15] On August 23, 2024, Thornwell signed with Zastal Zielona Góra of the Polish Basketball League (PLK).

[17] On February 11, 2025, Thornwell joined the Xinjiang Flying Tigers of the Chinese Basketball Association.