He was first elected to the Manitoba legislature in the 1915 provincial election,[3] defeating Conservative W.H. Sharpe by 84 votes in the constituency of Manitou. The Liberals won a landslide majority in this election, and Armstrong served as a backbench supporter of Tobias Norris's government for the next five years.
He ran for re-election in the 1920 campaign[3] but finished third against Conservative John Ridley, losing by 284 votes.
In 1929, Armstrong was named county court judge for the southern judicial district of Manitoba.[2]
He died in Morden after an extended illness at the age of 60.[2]