Before entering the league, Eichel was described at the age of 17 as "the new face of American hockey," and he was considered a member of a rising class of generational talents in the sport.

[2][3] Eichel was the recipient of the 2015 Hobey Baker Award, given to the top National Collegiate Athletic Association men's ice hockey player.

[8] After his second season within the Development Program, Eichel signed a letter of intent to commit to Boston University of the Hockey East on April 29, 2014.

The Boston Terriers reached the NCAA Frozen Four, where they lost to the Providence Friars 4–3 in the championship game, with Eichel recording an assist.

"[32] Specifically, Eichel was frustrated with the Sabres' refusal to clear him to undergo artificial disk replacement, a procedure recommended by his personal chiropractor and an outside doctor he had consulted, but had never before been performed on an active NHL player.

This was Eichel's first NHL game in almost a year, and he remarked that "I was having a pretty fun time out there," adding that he expected returning to competition would be a process.

[38] On March 10, he made his first appearance with the Golden Knights in Buffalo, where he was greeted with consistent booing from the crowd, notably when Tuch, who had been part of the trade, stripped the puck from him and scored to complete the Sabres' 3–1 victory.

[42] The 2022–23 season would begin on a much better note for the Golden Knights, with new coach Bruce Cassidy's arrival shifting the team's playing style and calling on Eichel to assume heavy responsibilities in the defensive zone.

[43] His second return to Buffalo would be a much greater success than the first, scoring a hat trick and also managing an assist while leading the Golden Knights to a 7–4 victory.

[44][45] Eichel missed several games in December and January with a lower body injury, which inaugurated a noticeable cold spell in his point production, in turn contributing to a downtick in the Golden Knights' record heading into the All-Star break.

[46] The Golden Knights qualified for the 2023 Stanley Cup playoffs with a first-round matchup against the Winnipeg Jets, ending Eichel's long history of missing the post-season.

[51] Vegas advanced to the second round to face the Edmonton Oilers, a matchup that attracted media attention due to its pitting Eichel against McDavid, which had been considered a potential career rivalry at the time of the 2015 draft.

[52][53][54] The Golden Knights defeated the Oilers in a six-game series, reaching the Western Conference Final against the Dallas Stars, with Eichel earning praise for his playoff performance.

At the conclusion of his freshman season with the Terriers, Eichel was named to make his full international debut with Team USA at the 2015 World Championships.

[64] Eichel scored 2 goals, including a game winner in the group stage against Slovakia, and 5 assists during the tournament, where Team USA won bronze.

On April 19, 2019, Eichel was selected to represent Team USA at the 2019 IIHF World Championship, held in Bratislava and Košice, Slovakia.