Reginald William Mountain (1899–1981) was a British civil engineer. Mountain was born in London in 1899. He served as an officer of the British Army's Royal Engineers during the First World War. From 1919 to 1922 Mountain studied for a Bachelor of Science degree in engineering at , graduating with a degree awarded by London University. He left the regular army and was placed in the Royal Engineers Special Reserve of Officers on 1 July 1921. At that point he held the rank of Second Lieutenant with seniority of 16 April 1921. He remained in the reserve and was promoted to Lieutenant on 7 August 1925 with his seniority backdated to 16 April 1923. Mountain left the reserve and resigned his commission on 15 November 1930.