He was promoted to lieutenant on 9 December 1910. The regiment was stationed at Risalpur in 1914 for service, if needed, on the North West Frontier. On the outbreak of the First World War in 1914 the officer who held the appointment of adjutant was posted away and Mayne was temporarily appointed to the role. On 8 May 1915 the appointment was made permanent. He saw action against the Mohmands and the Swatis on the North West Frontier between August and October 1915, was promoted to temporary captain on 1 September 1915 and having his first taste of staff work being appointed temporary Staff Captain in India from 23 to 30 September 1915. The regiment was sent to Mesopotamia in July 1916, Mayne arriving on 25 August. He was to serve in Mesopotamia until the end of the war against Turkey on 31 October 1918 and was wounded.
He was promoted to substantive captain during this period, his seniority later being antedated to 1 September 1915. He was made a companion of the Distinguished Service Order for service in Mesopotamia in the London Gazette, 25 August 1917.[4] He was additionally mentioned in despatches in the London Gazettes of 15 August 1917, 18 February 1919 and 5 June 1919.
He was first appointed to the staff as a General Staff Officer III (GSO3) on 27 March 1918[2] on the staff of the 3rd Army Corps, Mesopotamian Expeditionary Force remaining there until 11 September 1918. He was then appointed a temporary major on the staff of the General Officer Commanding (GOC) the Cavalry Division, Mesopotamia Expeditionary Force from 26 September to 31 December 1918, then a GSO II[2] with G.H.Q., Mesopotamian Expeditionary Force 1 January to 8 June 1919. He was appointed a brevet major (London Gazette 3 June 1919) for his services in Mesopotamia.
Between the wars
He was posted back to India as a Deputy Assistant Quartermaster General Army Headquarters India from 9 June 1919 to 15 May 1920.[2] He then went to England where he attended the Staff College, Camberley from 1921 to 1922.[2] His regiment amalgamated with the 16th Cavalry in 1921 to form the 6th Duke of Connaught's Own Lancers (Watson's Horse). He was appointed a General Staff Officer 2nd grade 15 February 1922 to 11 June 1923 at Army Headquarters followed by appointment as an instructor at the Cavalry School, Saugor from 12 December 1923 to 10 November 1924.[2] He was on leave on a medical certificate for one year to 19 September 1925. He served as a General Staff Officer 2nd grade at the War Office in London from 22 January 1927 to 22 January 1931.[2] Promoted to Brevet Lieutenant Colonel on 1 July 1930, he attended the Imperial Defence College in 1933 and transferred to the 9th Royal Deccan Horse on 29 August 1933 as second in command and then was appointed lieutenant colonel and commanding officer (CO) of the regiment from 4 August 1934 to 2 June 1936.[2][5]
He was promoted to local Brigadier whilst officiating as Director Military Operations & Intelligence 18 March 1936 to 3 June 1936 and temporary Brigadier whilst appointed as Director Military Operations & Intelligence from 3 June 1936 to 17 September 1938,[2] and retained the temporary rank when appointed to command the 4th (Secunderabad) Cavalry Brigade on 17 September 1938.[2] The 4th (Secunderabad) Cavalry Brigade was disbanded in July 1940 and he was appointed commander of the 9th Indian Infantry Brigade.[2][3]
Mayne was married in 1916 to Phyllis Tweddell who died in 1949. His only son pre-deceased him, killed in action in 1943 in the Italian campaign. He was friends with Indian army officers like Joyanto Nath Chaudhuri and actively promoted them to higher positions.[7] He died on 17 December 1955.
