In 1748 Thurlow entered Caius College, Cambridge, but an act of insubordination necessitated his leaving Cambridge without a degree (1751). He was for some time articled to a solicitor in Lincoln's Inn along with the poet Cowper, but in 1754 was called to the bar at the Inner Temple, and subsequently went on the western circuit, at first with little success. But in the case of Luke Robinson v. The Earl of Winchelsea (1758) Thurlow came into collision with Sir Fletcher Norton, afterwards 1st Baron Grantley (1716-1789), then the terror of solicitors and the tyrant of the bar, and put down his arrogance with dignity and success.
From this time his practice increased rapidly. In 1761 he was made a King's Counsel, through the influence of the Duchess of Queensberry. In 1762 he was elected a bencher of the Inner Temple. Thurlow now with some hesitation entered himself into the ranks of the Tory party. In 1768 he became member for Tarnworth. In 1769 the Douglas peerage case came on for hearing in the House of Lords, and Thurlow, who had drawn the pleadings some years before, led for the appellant in a speech of great analytic power. In 1770, as a recognition of his defence in the previous January of the expulsion of Wilkes, Thurlow was made Solicitor-General on the resignation of Dunning, and in the following year, after he had enhanced his reputation with the government by attacking the rights of juries in cases of libel and the liberty of the press, was raised to the Attorney-Generalship.
Thurlow's public life was as factious as his youth had been daring. His hatred of the American colonists, and his imprudent assertion that as Attorney-General he might set aside by scire facias as forfeited every charter in America; his speech in aggravation of punishment in the case of Home Tooke, when he argued that the prisoner ought to be pilloried, because imprisonment was no penalty to a man of sedentary habits and a fine would be paid by seditious subscription; and his opposition to all interference with the slave trade are characteristic.
In 1778 Thurlow became Lord Chancellor and Baron Thurlow of Ashfield, and took his seat in the House of Lords, where he soon acquired an. almost dictatorial power. He opposed the economical and constitutional reforms proposed by Burke and Dunning. Under Rockingham he clung to the chancellorship, while conducting himself like a leader of the opposition. To the short-lived ministry of Shelburne he gave consistent support. Under the coalition of Fox and North (April to December 1783) the Great Seal was placed in commission, and Lord Loughborough was made first commissioner. But Thurlow, acting as the kings adviser, and in acccordance with his wishes, harassed the new ministry, and ultimately secured the rejection of Fox's India Bill. The coalition was at once dissolved. Pitt accepted office, and Thurlow again became lord chancellor (23 December 1783).
At first he supported the government, but soon his overbearing temper asserted itself. Imprudently relying on the friendship of the king, and actuated by scarcely disguised enmity to Pitt, Thurlow passed rapidly from occasional acts of hostility to secret disaffection, and finally to open revolt. He delivered himself strongly against a bill, introduced without his privity, for the restoration to the heirs of attainted owners of estates forfeited in the Jacobite rebellion of 1745. Partly to please the king and queen, partly from dislike to Burke, and partly perhaps from a real belief in the groundlessness of the accusation, he supported Warren Hastings on every occasion with indecorous violence. His negotiations with the Whigs during the discussion of the Regency Bill (1788-19 February 1789) were designed to secure his seat on the woolsack in the event of Fox being called to power. The climax was reached in 1792, when he attacked Pitt's bill to establish a sinking fund for the redemption of the national debt, not on account of the economic objections to which it was liable, but on the trivial ground that it was an unconstitutional attempt to bind further parliaments. The bill was carried, but only by a narrow majority, and Pitt, feeling that co-operation with such a colleague was impossible, insisted successfully on his dismissal (15 June 1792).
The ex-chancellor, who had a few days before been created Baron Thurlow of Thurlow, with remainder to his brothers and their male descendants, now retired into private life, and, with the exception of a futile intrigue, under the auspices of the Prince of Wales, for the formation of a ministry from which Pitt and Fox should be excluded, and in which the Earl of Moira should be premier and Thurlow chancellor (1797), finally abandoned hope of office. In 1795 he opposed the Treason and Sedition bills without success. In 1801 he spoke on behalf of Home Tooke, now his friend, when a bill was introduced to render a priest in orders ineligible for a seat in the House of Commons. His last recorded appearance in the House of Lords was in 1802. He now spent his time between his villa at Dulwich and various seaside resorts. He died at Brighton on 12 September 1806, and was buried in the Temple church. Thurlow was never married, but left three natural daughters, for whom he made a handsome provision. The title descended to his nephew, son of the Bishop of Durham.
Lord Thurlow was a master of a coarse caustic wit, which habitually in his private and too frequently in his public life displayed itself in profanity. He was a good classical scholar and made occasional translations in verse from Homer and Euripides. His judicial and his ecclesiastical patronage were wisely exercised; he was the patron of Dr Johnson and of Crabbe, and was the first to detect the great legal merits of Eldon.
Thurlow's personal appearance was striking. His dark complexion, harsh but regular features, severe and dignified demeanour, piercing black eyes and bushy eyebrows, doubtless contributed to his professional and political eminence and provoked the sarcasm of Fox that he looked wiser than any man ever was. Yet he was far from being an impostor. By intense though irregular application he had acquired a wide if not a profound knowledge of law. Clear-headed, self-confident and fluent, able at once to reason temperately and to assert strongly, capable of grasping, rapidly assimilating, and forcibly reproducing minute and complicated details, he possessed all the qualities which command success. His speeches in the trial of the Duchess of Kingston for bigamy are vigorous and effective, while his famous opening in the Douglas peerage case and his argument for the Crown in Campbell v. Hall show that he might have rendered high service to the judicial literature of his country had he relied more upon his own industry and less upon the learning of Hargrave and Kenyon.