[2] He served as an officer of the British Army's Royal Engineers during the First World War.
[3] He left the regular army and was placed in the Royal Engineers Special Reserve of Officers on 1 July 1921.
[4] He remained in the reserve and was promoted to Lieutenant on 7 August 1925 with his seniority backdated to 16 April 1923.
[3] By 1931 Mountain had become an Associate Member of the ICE and was involved in hydro-electric energy and electricity systems.
Mountain continued to publish academic papers on subjects relating to hydroelectricity including a description of the method of electricity transmission used by the Central Electricity Board in Scotland and economic aspects of hydroelectric developments.