Kornfield started in January 2008 at the US Championships in Houghton for the first time in the US Super Tour and finished 102nd place over 10 km freestyle. At the Nordic Junior World Ski Championships 2010 in Hinterzarten , he achieved 77th place over 10 km classic and at the Nordic Junior World Ski Championships 2011 in Otepää, 56th place in the skiathlon and tenth place with the relay. In January 2012 he became the US sprint champion in Rumford , winning his first US Super Tour victory. At the U23 World Championships in 2014 in Fiemme Valley , he came in 19th place in the sprint. In the 2016/17 season he reached eight top ten finishes on the US Super Tour. He came in first, second and third place each time and ended the season in fourth place overall. In January 2018 he became the US American champion in the 30 km mass start race in Anchorage and took 38th place in the sprint in his World Cup debut in Planica . At the 2018 Winter Olympics in Pyeongchang , he ran 74th in the 15km freestyle and 48th in the 50km mass start race.
United States