 Mansfield was made king's counsel on 24 July 1772, and a bencher of the Middle Temple shortly after, on 6 November 1772.He was involved in the trials, in 1776, of the Duchess of Kingston for bigamy, and of Smith, Hollis, Calthorpe, and Beckford (candidates for Hindon) for bribery.In 1777, he was defence counsel for John the Painter, and crown prosecutor in 1779 against the counsellors who arrested Lord Pigot and took over Fort St George.