Thomas was born Feb. 7, 1989 in Tacoma, Washington. He's the son of James Thomas and Tina Baldtrip, and he was named after Hall of Fame point guard Isiah Thomas. He attended Curtis Senior High School in University Place, Washington through his junior year. As a junior, he averaged 31.2 points per game. He also attended South Kent School in South Kent, Connecticut and graduated from the prep school in 2008. For college, he attended the University of Washington. There, he would be named to the All-Pac-12 First Team in both 2010 and 2011. He was also named Pac-12 Tournament Most Outstanding Player both seasons. That was after being named Pac-12 Freshman of the Year. Thomas' No. 2 jersey is retired by the Huskies. He entered the NBA Draft following his junior campaign and was selected 60th overall by the Kings in 2011.