Montgomery signed a two-year deal and will replace Marc Savard on the Blues' bench.
"My family and I are very grateful to Mr. (Tom) Stillman (owner), Doug (Armstrong, general manager), and Craig (Berube, head coach) for this opportunity to be part of the Blues organization," Montgomery said. "This is an extremely talented group of players and staff and I am looking forward to assisting in any way to help the Blues regain the Stanley Cup."
The Stars fired Montgomery earlier this season for unprofessional conduct. The 51-year-old later admitted he checked into rehab for alcohol addiction less than a month after his dismissal.
Montgomery coached 113 games with Dallas, leading the club to a 63-40-10 record over parts of two seasons. Prior to securing his first NHL coaching job with the Stars, he led the University of Denver to a national championship.