Armstrong was appointed to the
Ontario Court of Appeal on January 25, 2002. In October 2002, Armstrong was awarded a
Doctor of Law (LLD)
honorary degree by the Law Society of Upper Canada for his contributions to Canadian law. In December 2008, Armstrong wrote the majority decision upholding a lower court's decision that unions cannot force courts to enforce fines it imposes on its members who cross
picket lines. 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. The union in the case applied for
leave to appeal the case to the
Supreme Court of Canada, but the application was dismissed with costs. 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. Armstrong wrote that this was because the defendant's actions – jumping out of a bush at the complainant and chasing her down a street – were "highly threatening and persistent" and had caused the
complainant to reasonably fear for her safety. The defendant's application for leave to appeal to the Supreme Court was dismissed. In February 2013, Armstrong, supported by the other two Court of Appeal judges hearing the case, dismissed the appeal in
R v Fearon. Armstrong wrote that Fearon's right to privacy was not violated because a cellphone without password protection can be searched by police during an arrest without a
warrant. Armstrong's decision was appealed to the Supreme Court, which dismissed the appeal in a 4–3 decision in 2014. Armstrong became a
supernumerary judge for the court in September 2012, and was replaced as a full-time judge by
Peter Lauwers from the
Ontario Superior Court of Justice. Armstrong retired in March 2013 upon reaching the mandatory retirement age of 75. == Post-judicial career ==