Law was born at Kings Cliffe, Northamptonshire, in 1686. In 1705 he entered Emmanuel College, Cambridge as a sizar; in 1711 he was elected fellow of his college and was ordained. He resided at Cambridge, teaching and taking occasional duty until the accession of George I, when his conscience forbade him to take the oaths of allegiance to the new government and of abjuration of the Stuarts. His Jacobitism had already been betrayed in a tripos speech. As a non-juror, he was deprived of his fellowship.
For the next few years Law is said to have been a curate in London. By 1727 he lived with Edward Gibbon (1666–1736) at Putney as tutor to his son Edward, father of the historian, who says that Law became the much-honoured friend and spiritual director of the family. In the same year he accompanied his pupil to Cambridge and lived with him as governor, in term time, for the next four years. His pupil then went abroad but Law was left at Putney, where he remained in Gibbon's house for more than 10 years, acting as a religious guide not only to the family but to a number of earnest-minded people who came to consult him. The most eminent of these were the two brothers, John and Charles Wesley, William Wilberforce, John Byrom the poet, George Cheyne the physician and Archibald Hutcheson, MP for Hastings.
The household dispersed in 1737. Law by 1740 retired to Kings Cliffe, where he had inherited from his father a house and a small property. There he was joined by Elizabeth Hutcheson, the rich widow of his old friend (who recommended on his death-bed that she place herself under Law's spiritual guidance) and Hester Gibbon, sister to his late pupil. For the next 21 years, the trio devoted themselves to worship, study and charity, until Law died on 9 April 1761.