After practicing for some years as a conveyancer, he was called to the bar at Lincoln's Inn in 1807, having already published his well-known treatise on the Law of Vendors and Purchasers. In 1822 he was made King's Counsel He was returned at different times for various boroughs to the House of Commons, where he made himself prominent by his opposition to the Reform Bill of 1832. He was appointed Solicitor-General in 1829, named Lord Chancellor of Ireland in 1834, and again from 1841 to 1846. In Lord Derby's first government in 1852 be became Lord Chancellor and was raised to the peerage as Lord St Leonards. In this position he devoted himseif with energy and vigour to the reform of the law; Lord Derby on his return to power in 1858 again offered him the same office, which from considerations of health he declined. He continued, however, to take an active interest especially in the legal matters that came before the House of Lords, and bestowed his particular attention on the reform of the law of property. He died at Boyle Farm, Thames Ditton.
After his death his will was missing, but his daughter, Charlotte Sugden, was able to recollect the contents of a most intricate document, and in the action of Sugden v. Lord St Leonards (L.R. 1 P.D. 154) the court accepted her evidence and granted probate of a paper propounded as containing the provisions of the lost will. This decision established the proposition that the contents of a lost will may be proved by secondary evidence, even of a single witness.
Lord St Leonards was the author of various important legal publications, many of which have passed through several editions. Besides the treatise on purchasers already mentioned, they include Powers, Cases decided by the House of Lords, Gilbert on Uses, New Real Property Laws and Handybook of Property Law, Misrepresentations in Campbells Lives of Lyndhurst and Brougham, corrected by St Leonards.