He was the commanding officer of the 5th South African Brigade in the Western Desert Campaign in North Africa during World War II.

[2] General Armstrong joined the Natal Police in 1910 and was transferred to the South African Mounted Rifles in 1913.

[1] He served in the British Army during World War I, but returned to the Union Defence Force in 1919.

In 1923 he was shifted to the Garrison Artillery, and became the commanding officer in 1933.

[1][3] He later served as Quartermaster-General and Chief of the General Staff.