In December 1818, Copley was made King's Serjeant and Chief Justice of Chester. He became Solicitor General on 24 July 1819 and was knighted in October, became Attorney General in 1824, Master of the Rolls in 1826 and Lord Chancellor in 1827. On his appointment to the latter post he was raised to the peerage as Baron Lyndhurst, of Lyndhurst in the County of Southampton. As solicitor-general he took a prominent part in the trial of Queen Caroline and was opposed to the Liberal measures which marked the end of the reign of George IV and the beginning of that of William IV. He was Lord Chief Baron of the Exchequer from 1831 to 1834. During the Melbourne government from 1835 to 1841 he figured conspicuously as an obstructionist in the House of Lords. His former adversary Lord Brougham, disgusted at his treatment by the Whig leaders, soon became his most powerful ally in opposition. Lyndhurst held the chancellorship from 1827–1830, 1834–1835, and 1841–1846. As he was in regard to Catholic emancipation, so in the agitation against the Corn Laws, he opposed reform until Peel, his chief, gave the signal for concession. In 1837 he was Rector of Marischal College (later Aberdeen University).