Pratt was admitted on 18 November 1675 a student at the Inner Temple, where he was called to the bar on 12 February 1682. He appeared for the Crown before the House of Lords in Sir John Fenwick's case, 16–17 December 1696, and before the House of Commons for the East India Company in support of the petition for a charter on 14 June and 1 July 1698. He was made serjeant-at-law on 6 November 1700, and was heard by a committee of the House of Commons as counsel for the court of exchequer against a bill for curtailing the fees of the officers of that court on 25 February 1706.