He served in the Legislative Assembly of Manitoba from 1907 to 1922 as a member of the Liberal Party, and was a cabinet minister in the government of Tobias Norris.
Armstrong worked as a physician and surgeon, and was also president and director of the Gladstone Telephone and Electric Light Company.
[1] The Conservative administration of Rodmond Roblin, which had governed Manitoba since 1900, was forced to resign from office in early 1915 as the result of a corruption scandal.
[4] Although the Liberals held a minority of seats in the legislature, they were called on by the province's Lieutenant Governor to form a new government.
Armstrong defeated Farmer candidate Wesley Lobb by 180 votes, and was retained in office after the election.