Armstrong, currently in his 10th season with the club, originally joined the Blues as the Vice President of Player Personnel in 2008 and was named the 11th General Manager in Blues history during the 2010 off-season. Since taking the reins, Armstrong has led the Blues to a 337-185-56 regular season record which serves as the fourth-best mark in the NHL since the 2010-11 campaign. He has also guided the team to six consecutive postseason berths, making the Blues one of only four teams to have accomplished that.

In his second season as General Manager, Armstrong captured the NHL’s 2012 General Manager of the Year Award after leading the club to a 49-22-11 record and its first Central Division title in 12 seasons. During his tenure, Armstrong has extended the contracts of core players such as Jake Allen, Patrik Berglund, Colton Parayko, Alex Pietrangelo, Jaden Schwartz, Alexander Steen and Vladimir Tarasenko. He has also orchestrated the acquisitions of Jay Bouwmeester and Brayden Schenn, signed free agents such as Paul Stastny and overseen the drafts of several prospects, including Vince Dunn and Tage Thompson.

Prior to joining the Blues, Armstrong spent 17 years with the Dallas Stars, including his final six as the team's General Manager. While with the Stars, Armstrong helped lead the franchise to two Presidents' Trophies, two Western Conference titles and the 1999 Stanley Cup.

Internationally, the Sarnia, Ontario, native has served Team Canada in numerous capacities, including Assistant General Manager at the 2002 and 2008 World Championships and General Manager at the 2009 World Championship. Armstrong also captured an Olympic gold medal as the Associate Director of Player Personnel in 2010 and as a member of the management group in 2014. Most recently, he served as General Manager of the gold medal-winning team at the 2016 World Cup of Hockey. Armstrong is a member of the "Triple Gold Club," having been on the management staff for a Stanley Cup, Olympic gold medal and World Championship gold medal.