Thorbjornsen qualified for the 2019 U.S. Open by winning the 2018 U.S. Junior Amateur at Baltusrol Golf Club. At the U.S. Open in 2019 he made the cut and ultimately finished in 79th place. He has had a number of junior tournament wins, including the PING Invitational and the U.S. Junior Amateur Invitational in 2018. In 2017, he came in fifth at the Massachusetts Open Championship and finished T5 at Rolex Tournament of Champions. This is his third appearance in the Junior Invitational (2018-20). He plans to play at Stanford University in the fall.