Mansergh was born on 14 May 1882, the son of Charles Henry Folliott and Elizabeth Ethel Fowkes. His unusual first name was his mother’s maiden name. He was educated at Clifton College. On 5 January 1901, he had already enlisted with the army, and in the 1901 census was stationed as a Lieutenant at Farnborough with the District 3rd Battalion of the Manchester Regiment. On 1 December 1912, he was promoted to Captain. He served in the South African War 1901 – 1902 and in East Africa 1908 – 1910.
When the First World War broke out, he served with the Expeditionary Force in France and Flanders. He was wounded, and died on 30 August 1914. He is remembered at the La Ferte-Sous-Jouarre Memorial in France.