His political career began in 1742 with his election as Member of Parliament (MP) for Boroughbridge and his appointment as Solicitor General. During the next fourteen years he was one of the most conspicuous figures in the parliamentary history of the time. By birth a Jacobite, by association a Tory, he was nevertheless a moderate, and his politics were really dominated by his legal interests. Although holding an office of subordinate rank, he was the chief defender of the government in the House of Commons, and during the time that William Pitt the Elder was in opposition had to bear the brunt of his attacks. In 1754 he became Attorney-General, and for the next two years acted as Leader of the House of Commons under the administration of Thomas Pelham-Holles, 1st Duke of Newcastle. But in 1756, when the government was evidently approaching its fall, an unexpected vacancy occurred in the chief justiceship of the king's bench, and he claimed the office, being at the same time raised to the peerage as Baron Mansfield.
From this time the chief interest of his career lies in his judicial work, but he did not wholly separate himself from politics. He became by a singular arrangement, only repeated in the case of Lord Ellenborough, a member of the cabinet, and remained in that position through various changes of administration for nearly fifteen years, and, although he persistently refused the chancellorship, he acted as Speaker of the House of Lords while the Great Seal was in commission. During the time of William Pitt the Elder's ascendancy he took but little part in politics, but while Lord Bute was in power his influence was very considerable, and seems mostly to have been exerted in favour of a more moderate line of policy. He was on the whole a supporter of the prerogative, but within definite limits. Macaulay terms him, justly enough, "the father of modern Toryism, of Toryism modified to suit an order of things in which the House of Commons is the most powerful body in the state."
During the stormy session of 1770 he came into violent collision, with Pitt the Elder and Charles Pratt, 1st Earl Camden in the questions that arose out of the Middlesex election and the trials for political libel; and in the subsequent years he was made the subject of the bitter attacks of Junius, in which his early Jacobite connexions, and his apparent leanings to arbitrary power, were used against him with extraordinary ability and virulence.
In 1776 he was created Earl of Mansfield. In 1783, although he declined to re-enter the cabinet, he acted as Speaker of the House of Lords during the coalition ministry, and with this his political career may be said to have closed. He continued to act as chief justice until his resignation in June 1788, and after five years spent in retirement died on 20 March 1793. He left no family, but his titles of 1776 and 1792 devolved upon his nephew's wife and his nephew, David Murray, 7th Viscount Stormont (1727–1796), respectively. Stormont became ambassador to Vienna and then to Paris; he was Secretary of State for the Southern Department from 1779 to 1782, and Lord President of the Council in 1783.
Lord Mansfield's great reputation rests chiefly on his judicial career. The political trials over which he presided, although they gave rise to numerous accusations against him, were conducted with singular fairness and propriety. He was accused with especial bitterness of favouring arbitrary power by the law which he laid down in the trials for libel which arose out of the publications of Junius and John Horne Tooke, and which at a later time he reaffirmed in the case of the dean of St Asaph. But we must remember that his view of the law was concurred in by the great majority of the judges and lawyers of that time, and was supported by undoubted precedents. In other instances, when the government was equally concerned, he was wholly free from suspicion.
He supported Lord Camden's decision against general warrants, and reversed the outlawry of John Wilkes. He was always ready to protect the rights of conscience, whether they were claimed by Dissenters or Catholics, and the popular fury which led to the destruction of his house during the Gordon riots was mainly due to the fact that a Catholic priest, who was accused of saying Mass, had escaped the penal laws by his charge to the jury. His chief celebrity, however, is founded upon the consummate ability with which he discharged the civil duties of his office.
He has always been recognized as the founder of English mercantile law. The common law as it existed before his time was wholly inadequate to cope with the new cases and customs which arose with the increasing development of commerce. The facts were left to the jury to decide as best they might, and no principle was ever extracted from them which might serve as a guide in subsequent cases. Mansfield found the law in this chaotic state, and left it in a form that was almost equivalent to a code. This is Mansfield on the legal nature of stolen banknotes when subsequently used as payment in the purchase of goods or services: "It is a pity that [court] reporters sometimes catch at quaint expressions that may happen to be dropped at the Bar or Bench; and mistake their meaning. It has been quaintly said, 'That the reason why money cannot be followed is, because it has no ear-mark:' but this is not true. The true reason is, upon account of the currency of it: it cannot be recovered after it has passed in currency. So, in case of money stolen, the true owner cannot recover it, after it has been paid away fairly and honestly upon a valuable and bona fide consideration: but before money has passed in currency, an action may be brought for the money itself." (Miller v Race (1758) 97 E.R. 398)
He defined almost every principle that governed commercial transactions in such a manner that his successors had only to apply the rules he had laid down. His knowledge of foreign and Roman law, and the general breadth of his education, freed him from the danger of relying too exclusively upon narrow precedents, and afforded him a storehouse of principles and illustrations, while the grasp and acuteness of his intellect enabled him to put his judgments in a form which almost always commanded assent. A similar influence was exerted by him in other branches of the common law; and although, after his retirement, a reaction took place, and he was regarded for a while as one who had corrupted the ancient principles of English law, these prejudices passed rapidly away, and the value of his work in bringing the older law into harmony with the needs of modern society has long been fully recognized.
"On the part of Somerset, the case which we gave notice should be decided, this day, the Court now proceeds to give its opinion. The state of slavery is of such a nature, that it is incapable of being introduced on any reasons, moral or political; but only positive law, which preserves its force long after the reasons, occasion, and time itself from whence it was created, is erased from memory: it's so odious, that nothing can be suffered to support it but positive law. Whatever inconveniences, therefore, may follow from a decision, I cannot say this case is allowed or approved by the law of England; and therefore the black must be discharged."
Mansfield concluded that there was no legal backing for slavery in England. Furthermore, the Somerset case is the origin of the following words about English common law (although Mansfield himself never said them) -- words that have been memorized by British pupils ever since.
Lord Mansfield is often misquoted as declaring that "The air of England is too pure for a slave to breathe", but no such words appear in the judgment. Instead, they appear repeatedly in the arguments of counsel for Somerset, who cited a report of a case from 1569, in the reign of Queen Elizabeth I, where it was reported that:
This ruling applied only to England, and not the rest of the British Empire, and British commerce in slaves continued for thirty-five years until 1807, when Parliament formally abolished the slave trade. From Lord Mansfield's ruling in this case comes also the famous quote, "Let Justice be done, though the Heavens may fall" which has been ascribed to Lucius Calpurnius Piso Caesoninus (d. 43 B.C.) in the form "Fiat justitia ruat coelum".
Since the ruling did not apply to British colonies, slavery remained in the future United States. However, the decision was used by American abolitionists to justify personal liberty laws, and was overturned in the United States by the 1843 Supreme Court decision of Prigg v. Pennsylvania.
Mansfield's nephew John Lindsay had a daughter, Dido Elizabeth Belle, whose mother was a slave, and Dido lived with Mansfield at Kenwood house. He was careful to confirm her freedom from slavery in his will in which he left her £100 a year.