Keller hasn't let a lack of size (5-foot-10, 170 pounds), keep him from making a big impact in the NHL. The native of Chesterfield, Missouri, is among the new generation of small, nimble forwards who have the skill to beat bigger opponents without taking a pounding.

After breaking Phil Kessel's record for points by a player in the USA Hockey National Team Development Program from 2014-16 (189), Keller was selected by the Arizona Coyotes in the first round (No. 7) of the 2016 NHL Draft. He played one season at Boston University, finishing with 45 points (21 goals, 24 assists), and helped the United States finish first at the 2017 IIHF World Junior Championship before signing with the Coyotes on March 26, 2017. Three days later, Keller had an assist in his second NHL game against the St. Louis Blues.

Keller scored his first NHL goal to open the 2017-18 season against the Anaheim Ducks and continued to pile up goals and points despite the Coyotes' slow start. He scored 11 goals in Arizona's first 16 games and reached the end of 2017 leading the Coyotes in goals, assists and points, although he was still more than six months away from his 20th birthday.

He finished his first NHL season with 65 points (23 goals, 42 assists) and was a finalist for the Calder Trophy, given to the top rookie in the NHL.

Keller signed an eight-year contract Sept. 4, 2019, that began in the 2020-21 season. He sustained a fractured femur on March 30, 2022. After surgery and six months of rehab, he responded by playing all 82 games in 2022-23, scoring 37 goals, 49 assists and 86 points. Keller completed his first NHL hat trick with 23 seconds left in overtime to give Arizona a 5-4 win against the Philadelphia Flyers at Mullett Arena on Dec. 11, 2022.

On Oct. 4, 2024, Keller was named captain of the Utah Hockey Club, the first in team history since the NHL Board of Governors approved the establishment of a franchise in Salt Lake City on June 13, 2024. He joined Pat LaFontaine (468 goals, 545 assists in 865 games) as the second Missouri-born player to reach the 500-point milestone in Utah's 4-1 win against the Winnipeg Jets at Delta Center on April 5, 2025.

Keller 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. His 10 points (three goals, seven assists) in 10 games helped the United States win the 2025 IIHF World Championship, its first gold medal in the tournament in 92 years.