John Law DD (1745–1810) was an English mathematician and clergyman who began his career as a Fellow of Christ's College, Cambridge, and went on to become chaplain to the Lord Lieutenant of Ireland and Church of Ireland bishop of Clonfert and Kilmacduagh (1782–1787), Killala and Achonry (1787–1795), and finally of Elphin (1795–1810).
He was a lifelong friend and correspondent of the philosopher William Paley.
The son of Edmund Law, later Bishop of Carlisle, Law was born at Greystoke in Cumberland, where his father was rector. He was educated at Charterhouse School and Christ's College, Cambridge, where in 1766 he graduated Bachelor of Arts with first-class honours in the Mathematical Tripos and was named as second Wrangler. He proceeded Master of Arts in 1769.
Law became a Fellow of Christ's and an Anglican clergyman, and spent several years as a tutor and lecturer at Cambridge. In 1773 his father gave him his first benefices, as vicar of Warkworth, Northumberland, and as prebendary of Carlisle. In 1777 he was collated Archdeacon of Carlisle. In April 1782, he went to Ireland as chaplain to the Lord Lieutenant of Ireland, William Cavendish-Bentinck, 3rd Duke of Portland, who had large estates in Cumberland.