After initially gaining experience in politics on the council of Eastnor Township, he relocated west to the city of Edmonton, Alberta.

In Edmonton, he established his pharmaceutical business once again and worked briefly with his brother on real estate and construction projects.

Approved by councils of both cities, a successful plebiscite was held, and bylaws permitting the merge were passed and Edmonton's annex of Strathcona came into effect in February 1912.

During his term, he introduced a new hospital, improvements to the street rail system, and commissioning a report on the expansion of the city.

In his early years in Edmonton, he also involved himself in several real estate ventures and construction projects along with his brother, Gordon.

[2] In 1906, at a citizen's meeting at a local store, Armstrong's name was speculated as a possible candidate for alderman for the upcoming 1906 election.

[14] In July of that same year, he would sign a bylaw that would allow an American entrepreneur to establish a gas plant in the city, an issue that had been presented to council for debate several months prior.

[18] Involved in national and provincial affairs, he voiced his support for a canal to be constructed in the Georgian Bay of Lake Huron, stating that the presence of such canal would improve the trading relations of Canada with the United States by allowing an easier route between the two countries.

[19][20] Additionally, during this first term as mayor, Armstrong also oversaw the city's festivities on George V's coronation and moved to form a municipal purchasing department.

[21][22] Another issue presented to council during his first term in office was that of potential amalgamation with the city south of the North Saskatchewan River, Strathcona.

[28] A plebiscite was held in both Edmonton and Strathcona on September 26, 1911,[29] with results at both municipalities overwhelmingly in favour of the joining of the two "twin cities".

[34] His opponent, Bryce J. Saunders charged that the city had long lacked efficiency in its utilities, particular the water supply, which he believed in the public ownership of.

[35] During his brief second term as mayor in 1912, Armstrong and council approved a plan to construct 110 miles of electric railway lines in the city,[36] considered legislation permitting band concerts on Sundays,[37] oversaw the opening of the Royal Alexandra Hospital,[38] advocated for the availability of housing accommodations for all Edmontonians in the face of an influx of new settlers,[39] and welcomed a visit from Governor General of Canada Prince Arthur, Duke of Connaught and Strathearn to the city, planning a grand celebration.

[40][41] It was also in 1912 that the Hudson's Bay Company put its Edmonton land holdings on the market, the first bridge across the North Saskatchewan River in the city's east end opened,[42] and that Armstrong commissioned a master plan for the newly amalgamated city, from an American firm based out of Minneapolis named Morell and Nichols.

[43] In his final week as mayor, the retiring Armstrong delivered the city's annual report at an annual meeting, thanked the people of Edmonton for entrusting him as mayor, and advised his successor to be fiscally responsible; stating, "If a reckless administration gets in next year, Edmonton will become a city like I was in recently, where they could not borrow enough money to clean their streets.

[47] In the same year he oversaw the construction of a new building on Namayo Avenue, the Fairbairn Block, mainly for use by his business but with additional space for offices and housing in the upper levels.

[51] Armstrong, who had proclaimed his innocence, and provided evidence for the trial was eventually acquitted and found not guilty of the charges in a decision made on May 29, which was followed by an eruption of applause.

The Edmonton Historical Board states that Armstrong was remembered as " hardworking, fair and honest person who served our city well during a time of rapid change and growth".