On the outbreak of World War I in 1914, he joined the British Army, from which he transferred to the Royal Flying Corps as a pilot, in 1915.

He fought in the skies over the Western Front in France and Belgium, and during 1916–17 he returned to South Africa on recruiting tours for the RFC.

1 & 2 in honour of the fact that they were purchased with money raised by the British community in Rio de Janeiro.

In 1936, Miller took part in the Portsmouth-to-Johannesburg Air Race, held to mark Johannesburg's Golden Jubilee.

During World War II, Miller served in the South African Air Force, where he commanded several flying schools.