Lehman won the senior 2012–2013 National Championship in the 5000 meters and three titles at the junior championships. He then won a gold and bronze medal at the World Junior Championships. In his senior year of high school, Lehman won the 10000 meters at the 2013–2014 US Senior Championship/Olympic Trials and took second place in the 5000 meters. At the Olympics, he placed 16th in the 5000 meters and 10th in the 10000 meters.
As of 2014, Lehman holds three national junior speed skating records.
Lehman began playing ice hockey when he was six years old. As a sophomore in high school, he was selected as a conference All-Star playing defense for Oak Park and River Forest High School's (OPRF) varsity team. Describing Lehman's game, his coach remarked "He delivers absolute monster hits ... He doesn't get knocked down, and he is always first to the puck". During the summer of 2013, Lehman hurt his ankle and the new OPRF hockey coach did not want to play him for fear of an injury jeopardizing Lehman's Olympic speed skating chances. Lehman thus took his senior year off from hockey.
At age nine, Lehman saw a poster claiming speed skating could help improve hockey skills. His mother persuaded Lehman to give it a try and he found he had a talent for the sport.
Lehman was a 2008–2009 short and long track speed skating national champion. He won the 5000 meters at the 2010–2011 United States Junior Championships and was third in the All-Around. He participated in the 2010–2011 World Junior Championships placing 11th in the 5000 meters and 18th in the All-Around, at age 14. At the 2011–2012 United States Junior Championships, Lehman won the 1500 meter, 3000 meter and 5000 meter distances, and also placed first in the All-Around. In his second year at the World Junior Championships, he was the youngest competitor in the field. At the senior National Championships that year, he placed sixth in the 1500 meter and 5000 meter events. He was fifth in the All-Around.
At the 2012 World Cup trials, Lehman improved his personal best in the 5000 meters by ten seconds, to 6:28.56. Aged 16, he became the youngest skater ever to break 6:30. He finished second overall and qualified for the senior World Cup team. "I know I'm doing well, but I didn't know it would be this well," Lehman remarked.
At the 2012–2013 US National Championships, he again improved his 5000-meter time, winning with a time of 6:27.06. He also took part in the 500, 1000, 1500 and 10000 meter events. He turned in personal best times in all four events and placed third in the 10000 meters. At the US Junior Championships, Lehman won the 1500, 3000 and 5000 meter events. At the World Junior Championships, he won a bronze medal in the 3000 meters and a gold medal in the 5000 meters with a time of 6:38.76. At the senior World Championships, he was placed 20th in the 5000 meters.
In November 2013, Lehman improved his own Junior National Record and came within 0.8 seconds of the World Junior record with a time of 6:19.87 in the 5000 meters. On December 27, Lehman, skating in the last of 10 pairs of athletes, completed the 5000 meters in a time of 6:25.90 at the United States Olympic Trials. The time placed him second and qualified Lehman for his first Olympic Games.
On January 1, 2014, Lehman took part in the 10000 meter race at the Olympic Trials. With three laps to go, he was four seconds behind 2010 Olympian Jonathan Kuck. Lehman took the lead with one lap to go. Kuck re-took the lead at the final turn, but Lehman edged him by seven hundredths of a second, about two blade lengths, at the finish line. "That was pretty insane," remarked Lehman. "I didn't expect to be finishing anywhere close to Jonathan". Lehman's time of 13:22.77 improved his personal best by seven seconds, and placed him first overall with one pairing – Patrick Meek and Edwin Park – remaining. Meek finished 0.39 seconds behind Lehman, taking third. Gary D'Amato of the Milwaukee Journal Sentinel called the race "perhaps the most thrilling race of the trials".
At the Olympics, Lehman placed 16th in the 5000 meters with a time of 6:29.94. He was the top finishing American. Later, he placed 10th in the 10000 meters with a time of 13:28.67, far exceeding his personal best for a race at sea level. The exertion of the race caused him to vomit. After the games, he went directly to the World Junior Championships.
Lehman trains at the Pettit National Ice Center and is coached by Eric Cepuran.
/> He is a member of the Franklin Park Speedskating Club.
Lehman also plays lacrosse competitively, serving as a midfielder. During the summers, he plays on an elite travel team in Illinois. As a freshman in high school, he was the team captain. Lehman's coach says he is a "very skilled player" who could play for a "high-level Division III to mid-level Division I" college program.
Lehman was born June 13, 1996, in Chicago, Illinois, to parents Marcia and David. As of 2014, he is a senior at Oak Park and River Forest High School, where he is an honor student. He is studying to become an engineer.
During his time off, Lehman enjoys playing table tennis with friends. He volunteers regularly at the Oak Park River Forest Food Pantry and is in the process of being certified as a speed skating coach. Lehman lists the Chicago Bears and Chicago Blackhawks as his favorite sports teams, and Bo Jackson as his sporting hero. He says his mother has been the most influential person in his life.