continued to represent Seaford until his retirement from parliamentary life in 1816. In Hilary term 1807 Leach was made a king's counsel, and was subsequently elected a bencher of the Middle Temple. Leach spoke but rarely in the House of Commons. In March 1809 he defended the conduct of the Duke of York (Parl. Debates, 1st ser. xiii. 289–99), and on 31 Dec. 1810 supported William Lamb's amendment to the first regency resolution (ib. xviii. 532–45). In 1811 he carried through the House of Commons the Foreign Ministers' Pension Bill (51 Geo. III, c. 21). On 15 Feb. 1813 he strongly protested against the bill for the creation of a vice-chancellor, the effect of which he maintained would be to make the lord chancellor a political rather than a judicial character (ib. xxiv. 519–31, 534); and on 31 May 1815 he strenuously opposed Lord Althorp's motion for an inquiry into the expenditure of 100,000l. granted by parliament for the outfit of the prince regent (ib. xxxi. 548–9).
Early in February 1816 Leach vacated his seat in the House of Commons by accepting the Chiltern Hundreds, and was immediately afterwards appointed by the prince regent chancellor of the Duchy of Cornwall. In August 1817 he became chief justice of Chester, in succession to Sir William Garrow. Resigning these posts, he succeeded Sir Thomas Plumer as vice-chancellor of England in January 1818, and having been sworn a member of the privy council on 30 Dec. 1817, was knighted in the following month. Upon Copley becoming lord chancellor Leach was appointed master of the rolls (3 May 1827), and, by a commission dated 5 May 1827, was made deputy-speaker of the House of Lords (Journals of the House of Lords, lix. 278). By an act of parliament passed in August 1833 (3 and 4 William IV, c. 41) Leach became, by virtue of his office as master of the rolls, a member of the judicial committee of the privy council. He died at Simpson's Hotel in Edinburgh on 14 Sept. 1834, aged 74, and was buried on the 20th of the same month in William Adam's mausoleum in Greyfriars churchyard (James Brown, Epitaphs in Greyfriars Churchyard, Edinburgh, 1867, pp. 200–1).
According to Romilly, Leach had ‘great facility of apprehension, considerable powers of argumentation, and remarkably clear and perspicuous elocution,’ but was extremely wanting in knowledge as a lawyer, and in judgment was ‘more deficient than any man possessed of so clear an understanding that I ever met with’ (Memoirs, iii. 216–17). Leach got through his cases with remarkable speed. The chancery court under Lord Eldon was called the Court of Oyer sans Terminer, and the vice-chancellor's the Court of Terminer sans Oyer. Leach's decisions were lucid and brief, but as he often decided on his own judgment in preference to that of his predecessors, they were not unfrequently overruled. They will be found in the ‘Reports’ of Buck, Glyn and Jameson, Maddock (vols. iii–vi.), Montagu and Macarthur (i. 1–8), Mylne and Keen, Russell, Russell and Mylne, Simons (i. 1–291), Simons and Stuart and of Tamlyn.
Leach's irritable temper and dictatorial demeanour on the bench brought him into constant collision with members of the bar. A deputation from the most distinguished counsel of his court is said to have done some good by a formal remonstrance (Legal Observer, viii. 452). During his vice-chancellorship his salary was raised to 6,000l., and that of the master of the rolls to 7,000l. a year (6 Geo. IV, c. 84, sec. 2). While he was master of the rolls the customary evening sittings of the court were abandoned, and on 22 June 1829 the practice of sitting in the daytime was commenced (Tamlyn, Reports, 1831, i. p. xiii). Though Leach was professedly a whig when he entered parliament, he adopted the politics of the regent, whose confidential adviser he had become. At his instigation the Milan commission was instituted in 1818 to investigate the conduct of the princess, but he did not, as it was sometimes asserted, prosecute the inquiry himself (Twiss, Life of Lord Eldon, 1844, ii. 400–2). He was strongly in favour of a divorce, and in April 1820 is said to have tried ‘to root out the ministry’ by telling the king that his ministers were not standing by him in the matter (Life of William Wilberforce, 1839, v. 54; see also Croker's Correspondence and Diaries, 1884, i. 160–1, and Lord Colchester's Diary, 1861, iii. 115). Leach appears to have aspired to the woolsack more than once, and in November 1830 was ‘exceedingly disappointed’ at Brougham's appointment (Greville Memoirs, 1st ser. 1874, ii. 68). In private life he is said to have been amiable and courteous. His manners were finical and affected. Ambitious ‘of being thought to unite the character of a fine gentleman to that of a great lawyer,’ he shunned the society of his own profession, and ‘was in constant attendance at the opera and at the gayest assemblies’ (Romilly, iii. 217). Leach was created D.C.L. by the university of Oxford on 5 July 1810. He was never married. His nephew, Richard Howell Leach, a son of his youngest brother, Thomas Leach, was the senior chancery registrar from 1868 to 1882, and