Lord Chief Justice, trustee of the Atholl settlement of the Isle of Man, and the man whose advice fixed the price at the Revestment.

His Manx involvement began long before the sale. By the indenture of feoffment of 14 November 1737, Duke James granted and confirmed the Island and lordship of Man to John, Earl of Dunmore, the Honourable William Murray, afterwards Lord Mansfield, and John Murray, on trusts reserved to the Duke to revoke.

He was therefore a trustee of the estate he later helped to sell, and a kinsman of the sellers. When the Duke and Duchess of Atholl came to terms with the Crown in 1765, they named seventy thousand pounds. Their own account, given to Parliament long afterwards, was that they named that figure because they understood from Lord Mansfield, their mutual friend, that no more would be given if they asked it, and that even then they would not have signed the contract had they not been promised the pension of two thousand a year as part of the bargain. The Duke's letter to Mansfield asking his advice enclosed a detailed estimate putting the family's losses at six hundred and twenty thousand pounds. That gap between the estimate and the price is the arithmetic on which the Atholl claims of 1791, 1805 and 1825 all rested.

In England he is the Mansfield of the general warrants and the law of libel. He confirmed Pratt's decision that general warrants were illegal. His doctrine that a jury had nothing to do with whether a paper was a libel was attacked by Fox in the Commons in 1791, who allowed that no man would dispute his integrity as a judge while holding that he had got into a position there was no defending. Wilkes, in exile, believed Mansfield would pass a terrible sentence on him.