He was first elected to the Manitoba legislature in the provincial election of 1892, winning without opposition in the Woodlands constituency. The Manitoba Liberal Party won a majority government in this election, and Armstrong sat with the small opposition group.
He resigned from the legislature in 1896 to run for the House of Commons of Canada as a candidate of the federal Conservative Party. He lost the riding of Selkirk to Liberal Party candidate John Alexander MacDonell by a single vote.
The Manitoba Conservative Party formed government in 1900, initially under Hugh John Macdonald and subsequently under Rodmond Roblin. Armstrong returned to the legislature in 1902, and the sitting member for Portage la Prairie died. He was again returned without opposition, and sat as a government backbencher.
Armstrong was re-elected in the 1903 election, defeating Liberal candidate Edward Brown by thirty-three votes. Brown became leader of the provincial Liberal Party in 1907, and again challenged Armstrong in that year's general election. Armstrong was again victorious, defeating Brown by 223 votes.
Armstrong entered Roblin's cabinet on November 19, 1908, in the senior portfolio of Provincial Treasurer. He served in this capacity for the remainder of his career in politics. In the 1910 election, he defeated Liberal challenger Ewan McPherson by 201 votes.
MacPherson challenged Armstrong again in the 1914 provincial election, and this time defeated him by ten votes. The Conservatives again won a majority government, and Armstrong returned to the legislature after winning a deferred election in the northern constituency of Grand Rapids.
In 1915, the Roblin ministry was forced to resign from office after a report commissioned by the Lieutenant Governor found the government guilty of corruption in the tendering of contracts for new legislative buildings. Armstrong resigned from office on May 12, 1915, and did not seek re-election in the 1915 campaign.
