His first term as mayor also saw the issue of possible amalgamation of the "twin cities" of Edmonton and Strathcona, an issue that had been spoken of as a possibility in the previous few years. 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. Armstrong put his name up once again in the mayoral election mandated by the agreement and defeated his rival candidate, and thus became the first post-amalgamation mayor of the city of Edmonton. During his term, he introduced a new hospital, improvements to the street rail system, and commissioning a report on the expansion of the city. Another election was held later that December, and he did not stand for re-election.