Son of Dr Joseph Forster Armstrong, medical doctor, and Sarah JaneArmstrong, Forster joined the High School in 1885.

In December, 1889, At the age of 13, he left to be a boarder at Giggleswick Grammar School in North Yorkshirem and went on to Christ's College, Cambridge to study law.

After graduating in 1899 with an LL.B in the Law Tripos he became a partner in his grandfather's firm of J M Moore & Armstrong.

Some years before the war he enrolled with the 3rd Durham Volunteer Artillery and received his commission in 1902. He was promoted to 2nd Lieutenant in February, 1903.

Forster was later promoted to Captain and, in April, 1908, the force was reconstituted as part of the 4th Northumbrian Howitzer Brigade, Royal Field Artillery

In August 1914 he was mobilised as a Captain in the 4th Northumberland Brigade of the Royal Field Artillery, and attached as adjutant to the Northumbrian Divisional Artillery Column three months later.

He was promoted to Major on 1-Jun-1916 and given command of a field artillery battery.

Initially he served at home, but on 21-Apr-1915 he was deployed to France.

He was killed in action (possibly by a stray shell) between St Martin's-sur-Cojeul and Heninel, just east of Arras, on 25-Sep-1917, and buried in the St. Martin Calvaire British Cemetery St. Martin-Sur-Cojeul.