Eichel's first trip to the Stanley Cup Playoffs ended with NHL-highs in assists (20) and points (26) and lifting the Cup after the Vegas Golden Knights defeated the Florida Panthers 9-3 in Game 5 of the Stanley Cup Final. Those 26 points were the third-highest total in NHL history by a player making his first trip to the postseason. He had three assists in Game 5 to finish behind only Mark Recchi (34 points for the 1991 Pittsburgh Penguins) and Eric Staal (28 for the 2006 Carolina Hurricanes).

Eichel also became the second United States-born player in the NHL modern era (since 1943-44) to lead the playoffs in points (Brian Leetch, 34 for the 1994 New York Rangers). He played for the United States at the 4 Nations Face-Off during the 2024-25 season, when he had an NHL career-high 94 points (28 goals, 66 assists), and won gold with Team USA at the Olympic Winter Games Milano Cortina 2026, the country's first in men's Olympic hockey since the 1980 Lake Placid Games. He signed an eight-year, $108 million contract with the Golden Knights (average annual value of $13.5 million ) on Oct. 8, 2025.

Selected by the Buffalo Sabres with the No. 2 pick in the 2015 NHL Draft, Eichel entered the League already experiencing plenty of success at a young age.

In 2014, he helped the United States win the gold medal at the 2014 World Under-18 Championship with 10 points (five goals, five assists) in seven games, then competed as a 17-year-old at the World Junior Championship as the youngest player for the United States.

The following season he played for Boston University, where he led all Division I players with 71 points (26 goals, 45 assists) and a plus-51 rating in 40 games. Eichel helped lead BU to the national championship game, where it lost 4-3 to Providence College.

Following the season, he won the Tim Taylor Award, player judged to be the most outstanding freshman, was named the Hockey East Rookie and Player of the Year, and was also named to the NCAA East First All-American Team.

On April 10, 2015, Eichel became the second freshman to win the Hobey Baker Award, voted the top men's player in NCAA ice hockey, joining Hockey Hall of Fame forward Paul Kariya, who won it with Maine in 1993.

That season, Eichel was also named captain for the United States at the 2015 IIHF World Junior Championship, where he had four points (one goal, three assists) in five games. He capped his busy season by skating for the United States the World Championship, winning the bronze medal and ranking third on the team with seven points (two goals, five assists) in 10 games.

Jumping straight into the NHL with the Sabres the following season, Eichel immediately showed his talent, scoring Buffalo's lone goal in his NHL debut, a 3-1 loss to the Ottawa Senators on Oct. 8, 2015. He finished the season with 56 points, including a team-high 24 goals, in 81 games and was named to the NHL All-Rookie Team.

Eichel scored 24 and 25 goals in his next two seasons despite being hampered by injuries, but the Sabres showed their faith in the then 21-year-old center by naming him captain on Oct. 3, 2018, exactly one year after agreeing to an eight-year, $80 million contract extension with Buffalo.

Eichel rewarded that faith by scoring 82 points (28 goals, 54 assists) in 77 games that season, but he was even better in 2019-20. He had 78 points (36 goals, 42 assists) in 68 games, the fourth straight season he led Buffalo in points, before the season was paused on March 12 due to concerns surrounding the coronavirus. As a result, he was named an all-star for the third straight season.

He was limited to 21 games in the 2020-21 season because of a neck injury, and was traded to the Golden Knights on Nov. 4, 2021.