Born at Alwalton, Huntingdonshire, a few miles from Peterborough, on 16 March 1729, he was the eldest of ten children of William Southgate (d. February 1771), a farmer of the parish, who married Hannah (d. 1772), daughter of Robert Wright of Castor, Northamptonshire, a surveyor and civil engineer.

Southgate took holy orders in 1752, and, after serving the curacy of Weston, Lincolnshire, held the rectory of Woolley, Huntingdonshire from 8 November 1754 till 1759.

On Christmas Day 1765 he accepted the same post at St Giles-in-the-Fields, London, and held it for the rest of his life.

Collections were made by him for a ‘History of the Saxons and Danes in England,’ illustrated by coins, but the work was not completed.

His coins and medals were announced for sale in eight days, but, according to John Nichols, they passed by private contract to Samuel Tyssen.