Poston was born in 1945 in St Albans, a city in southern Hertfordshire, England. He was the son of an Anglican priest and a social worker. He was educated at the Perse school in Cambridge before going on to attend Exeter University, where he studied French and history. After graduating in 1970, he joined the Foreign Office.
Poston served as the British consul general to New England from 1995 until 1999. While there, he arranged for the then Unionist leader David Trimble to visit and meet local Irish-Americans. He also served the Foreign Office in Brussels, Israel, Nigeria, and South Africa.