Born in Chesterfield, Missouri, and raised in Swansea, Illinois, Keller was part of a group of skilled young hockey players around his age growing up in the St. Louis area.

He recorded his 200th career point and received his second All-Star Game selection during the 2021–22 season, but suffered a season-ending leg injury at the end of March.

[7] In 2012, Keller joined the ice hockey team at Shattuck-Saint Mary's, a boarding school in Faribault, Minnesota, and the alma mater of NHL players Sidney Crosby, Jonathan Toews, and Kyle Okposo.

[13] He spent two seasons in the NTDP, with 71 goals and 118 assists for 189 career points, setting the program record until 2019, when he was surpassed by Jack Hughes.

[19] He was injured during the Terriers' 3–0 win over Northeastern at the start of November, with sophomore Jordan Greenway taking his place as Boston's second-line center in his absence.

[28] They received a bid in the 2017 NCAA Division I Men's Ice Hockey Tournament, facing reigning champions North Dakota in the first round.

[34] Keller recorded his first point in his first home game at Gila River Arena, assisting on Alex Goligoski's third-period goal against the Blues.

[36] After impressing new coach Rick Tocchet in training camp, Keller started the 2017–18 season as the right winger on the Coyotes' top offensive line.

[42] With one goal and three assists on February 15, Keller joined Martin Hanzal and Kyle Turris as the only Coyotes rookies to record a four-point game.

Those marks passed Peter Mueller and Max Domi, respectively, for the most of any franchise rookie since the Coyotes relocated from Winnipeg prior to the 1996–97 season.

[47] Keller finished third in voting for the 2018 Calder Memorial Trophy, given to the top NHL rookie, behind winner Mathew Barzal and runner-up Brock Boeser.

[58] While facing the Detroit Red Wings on December 22, Keller scored twice and assisted once in the Coyotes' 5–2 victory, becoming the first player in franchise history to record four three-point games before his 22nd birthday.

[63] One of eight Coyotes making his postseason debut in 2020, Keller led his team in scoring with four goals and seven points in nine playoff games before the Colorado Avalanche eliminated Arizona in the second tournament round.

[64] Keller dropped to the Coyotes' third line for the beginning of the 2020–21 season, playing on newcomer Derick Brassard's left wing with Tyler Pitlick on the right.

[68] In the seven consecutive games in which the Coyotes faced the Blues, the Short Leash Line combined for 21 points, with Keller registering three goals and three assists.

Keller reached the milestone in his 310th career game, becoming the 11th Coyote to register 200 points and the fifth member of the 2016 NHL Draft class to do so.

At the time of the injury, the Coyotes had been mathematically eliminated from playoff contention, but Keller had a team-leading 63 points, including a career-high 28 goals, in 67 games.

He was his team's representative in the 2023 All-Star Game,[78] and was named a finalist for the Bill Masterton Memorial Trophy, awarded to "the NHL player voted to best exemplify the qualities of perseverance, sportsmanship, and dedication to hockey.

[84] Although the United States only took bronze at the 2016 IIHF World U18 Championships, Keller had four goals and 14 points in seven games, and he was named the Tournament MVP and a member of the Media All-Star Team.

[88] Three Coyotes played for the United States men's national ice hockey team at the 2017 IIHF World Championship, including Keller.

[89] The youngest player in that year's tournament, Keller scored five goals in seven preliminary-round games,[90] including a hat-trick in Team USA's 7–2 victory over Denmark.