Originally undrafted by teams in the NHL, he played a total of six seasons for the Blues, Montreal Canadiens, Philadelphia Flyers, San Jose Sharks, and Dallas Stars.
The following season he joined the University of Maine and played four years with the team, winning numerous awards and establishing himself as one of the best prospects in ice hockey.
Montgomery is credited with nicknaming the dominant line of John LeClair, Eric Lindros, and Mikael Renberg the "Legion of Doom".
On April 12, 2010, he was named head coach of the United States Hockey League (USHL) expansion franchise Dubuque Fighting Saints.
In the team's first year, Montgomery guided the Fighting Saints to a 37–14–9 record and the 2010–11 USHL championship with a three games to one victory over the Green Bay Gamblers.
In 2013, Montgomery was signed by University of Denver as head coach of their Pioneers men's ice hockey team and led them to a berth in the NCAA tournament.
In 2017, his fourth year as the head coach of the Pioneers, he led them to the National Championship game after establishing them as the first-seeded team in the country for the majority of the season.
"[8] At a press conference, general manager Jim Nill said the situation had come to light the previous weekend, and involved "a material act of unprofessionalism" egregious enough to demand Montgomery's immediate firing.
[14] On September 16, 2020, the St. Louis Blues signed Montgomery to a two-year contract, serving as assistant coach under Craig Berube.
[16] Montgomery's "positive coaching" was widely assessed as a significant shift from his predecessor's approach, and led to major improvements in the team for the 2022–23 season.
The team received the Presidents' Trophy, while Montgomery won the Jack Adams Award as NHL coach of the year.
[23] On November 24, 2024, five days after his departure from Boston, Montgomery signed a five-year contract to return to the Blues as head coach, succeeding Drew Bannister.