He was called to the Bar on 15 May 1702, and in 1720 married Elizabeth Blencowe, daughter of Sir John Blencowe. On 3 November 1726 he became a Justice of the King"s Bench, succeeding Sir Littleton Powys, and on 24 November 1740 became Chief Baron of the Exchequer, succeeding Sir John Comyns.
After 20 years of "the usual forensic drudgery", Probyn secured appointment as a justice of the Brecon, Glamorgan, and Radnor circuit court in 1721, and became a Serjeant-at-Law on 27 January 1724, defending Thomas Parker, 1st Earl of Macclesfield at his trial for embezzlement.
This he did, and later sat as a Member of Parliament for Wootton Bassett.
