Before serving on the bench, Armstrong was a partner at Torys and was lead counsel in the Dubin Inquiry on steroid use in Canadian sports.

After leaving the bench, Armstrong joined Arbitration Place, a Canadian group specializing in alternative dispute resolution.

[7] Armstrong worked with his mentor Justice Charles Dubin, the chair of the inquiry, to guide the testimony of witnesses such as Ben Johnson, Angella Taylor-Issajenko, Charlie Francis about the extent of their anabolic steroid use and how they avoided discovery.

[10] He is also a former member of the Board of Regents of the American College of Trial Lawyers,[11] and the former president of the Canadian Institute for Advanced Legal Studies.

Armstrong, with Justice Paul Rouleau concurring, found that the union's constitution created a power imbalance with its members, making it unconscionable for the court to enforce the fines.

Armstrong's colleague, Justice Russell G. Juriansz, dissented saying that the imbalance of power had not been abused and the workers were always free to leave the union.

[17] In February 2009, Armstrong wrote a decision holding that a defendant had correctly been found guilty of criminal harassment even though there was no previous contact and there was only a single incident, unlike the repetitive nature of traditional stalking cases.