Armstrong was born in Maskinongé, the son of Edmond Armstrong and Elizabeth Dunn. His father had been a captain in the Royal Navy, serving during the American Revolutionary War, and then was the harbour master of Montreal. His mother's family were United Empire Loyalists who emigrated to Canada.[1] He was educated in part at the English school in Maskinongé.[2]
Around 1824, Armstrong moved with his parents to Berthier, where he established himself as a merchant, as well as an agent for insurance companies. He married Léocadie Deligny in 1831. She was the daughter of Jacques Deligny, who had been a member in the Legislative Assembly of Lower Canada for over twenty years, first for the district of Warwick, and then for Berthier.[1][3]
Armstrong was a justice of the peace and a commissioner for the trial of Small Causes. He entered the militia, eventually becoming lieutenant-colonel of the 4th Battalion of the Berthier Militia.[1]
His daughter Amélie married Michel Mathieu, who served in the Canadian House of Commons.
Armstrong stood for election in the first general election in 1841 and was elected to the Legislative Assembly of the new province, representing the electoral district of Berthier. He was re-elected in 1844 and 1848, but defeated in 1851.[2]
In the first session of the first Parliament, elected in 1841, Armstrong was a member of the French-Canadian Group from Canada East, who opposed both the new union of Canada, and the government of Governor General Sydenham. Following Sydenham's death in late 1841, he continued to oppose the government of the new Governor General, Sir Charles Bagot, until Bagot was forced to reorganise his government by taking in Reformers, led by Louis-Hippolyte Lafontaine and Robert Baldwin. Armstrong supported the new government, and also voted in opposition when the Lafontaine–Baldwin ministry felt compelled to resign.[2][7]
Armstrong subsequently became identified with the Reform group, until his defeat in 1851.[2]
In 1855, he was named to the Legislative Council of the Province of Canada for the Sorel division and served until Canadian Confederation in 1867, when the province of Quebec was created. He was named to the Legislative Council of Quebec in 1867, again for the Sorel division. By the time he sat in the Legislative Council of Quebec, he was considered a Conservative. He served in the Quebec Legislative Council from 1867 until his death in Sorel in 1873. He was entombed in the vault of the church of St Pierre at Sorel.[2]