‘Maunsell’, as he was known, was the only son of Robert John Maunsell Mayne, Police Superintendent in Madras, South India, and Mary Emily née Caldwell, from a Glasgow family with strong Irish roots. Maunsell was educated in Scotland where he was awarded an exhibition to Glenalmond School (1908-13). He became a professional soldier and was commissioned into the Royal Artillery from RMA Woolwich in September 1914. By the end of the year he was fighting in France and Belgium (1914-15 Star) and then in Italy (Despatches 1917, Military Cross 1918, Italian bronze medal for valour). He was married at Eastbourne in 1917 to Gwen Annesley Vaughan, the daughter of Ernest Llewelyn who had retired from the Indian Civil Service. Maunsell survived the war but suffered from its after-effects and disappeared without trace in 1927 on a train journey between London and East Anglia. He was presumed dead in 1938. He left a son, Peter Maunsell Mayne (born 1922) whose whereabouts are unknown.
You can read more at https://www.scribd.com/doc/75988391/Sedborough-Mayne-of-Ireland pages 36-37.