Thorbjornsen captured the 2018 U.S. Junior Amateur Championship at Baltusrol, 1 up, over Akshay Bhatia and then made the cut in the U.S. Open at Pebble Beach Golf Links the following year. He is the only Junior champion to play 72 holes under the new U.S. Open exemption that was started in 2018 by the USGA that grants the reigning Junior Am champ a spot in the field. Thorbjornsen qualified for his third U.S. Open by earning medaist honors at Canoe Brook Country Club with an 8-under total of 134 on the North and South courses. He also qualified for the 2022 U.S. Open at The Country Club in Brookline, Mass., and had the honor of hitting the opening tee shot, as he grew up a short drive away in Wellesley, Mass. Thorbjornsen just completed his junior season at Stanford University, where he won the Pacific-12 Conference title and helped the Cardinal reach the NCAA Championships in Scottsdale, Ariz. He represented the USA in the 2022 Palmer Cup. Thorbjornsen won the 2021 Western Amateur and Massachusetts Amateur, defeating 2017 U.S. Mid-Amateur champion Matt Parziale in the final match. In 2020, he reached the quarterfinals of the U.S. Amateur at Bandon Dunes with fellow Cardinal Karl Vilips serving as his caddie.