Armstrong replaced Bob Gainey as GM of the Stars with 32 games to go in the 2001–02 NHL season, and one of his first moves was hiring Dave Tippett as head coach.

On November 13, 2007, in the wake of a 7–7–3 start and a colossal meltdown by the team away against the Los Angeles Kings (losing 6–5 in overtime after leading 4–0 with seven minutes remaining in the game), Armstrong was fired as GM and replaced by former Stars player Brett Hull and assistant GM Les Jackson as interim co-general managers.

Some important moves Armstrong made during his time as GM included the drafting of players like Trevor Daley, James Neal, Loui Eriksson and Jamie Benn.

On June 17, 2010, the Blues acquired goaltender Jaroslav Halak from the Montreal Canadiens in exchange for prospects Lars Eller and Ian Schultz.

In 2018–19, Armstrong's Blues would surge up the standings for a third-place finish in the Central Division before eventually defeating the Boston Bruins in seven games to win their first Stanley Cup championship in franchise history.