De Roeper joined the Royal Naval Air Service on 16 April 1915 as a Temporary Flight Sub-Lieutenant, based at HMS President.

[12] On 1 August 1919 de Roeper received a full Royal Air Force commission in the rank of Major[13] (later converted to Squadron Leader), and the following day at Romford married Jean Julia Key, of Upminster, Essex.

[16] On 14 November 1924 de Roeper was proposed by William James Stewart Lockyer to be a Fellow of the Royal Astronomical Society.

[22] De Roeper rejoined the Royal Air Force in the rank of Squadron Leader during World War II, being transferred from the General Duties to the Technical Branch on 24 April 1940.

[23] He was promoted to Wing Commander on 1 January 1944,[24] and later to Temporary Group Captain, this being made war substantive on 24 March 1945.

[25] On 3 March 1946 de Roeper returned to the retired list at his own request, retaining the rank of Group Captain.