R. Mason Thomas attended Cardinal Gibbons High School in Fort Lauderdale, Florida, where he played football and ran track. Thomas collected a combined 109 tackles, 36 tackles for a loss, 19 sacks, and an interception over his junior and senior seasons. He was a four-star recruit and the No. 17 edge in the country when he committed to the University of Oklahoma.
In his first season at Oklahoma, Thomas played in 10 games and totaled seven tackles, one tackle for loss, and 0.5 sack. He played in nine games in a rotational role at defensive end during the 2023 season, however he missed four games during the season with a sprained ankle and finished with nine tackles, 2.5 tackles for a loss and one sack.