He was admitted a student of Lincoln's Inn on 6 November 1740, and on 27 April 1746 migrated to the Inner Temple, where he was called to the bar on 3 July 1747.
On 6 April 1776 he kissed hands on his appointment as a Baron of the Exchequer in the place of Sir John Burland, and was knighted on the same day.
He was buried on the 10th of the same month in ‘the new burial-ground’ at Twickenham, and a tablet was erected to his memory in the south chancel wall of the old parish church.
Some remarks on Perryn's charge to the grand jury of Sussex at the Lent assizes in 1785 are appended to Thoughts on Executive Justice with respect to our Criminal Laws, particularly on the Circuits, London, 1785.
Perryn married Mary, eldest daughter of Henry Browne of Skelbrooke in the West Riding of Yorkshire, by whom he had several children.