As must be the case with any court in central position, politics followed Mansfield to the bench. Three cases reveal his characteristic aloofness from personal or popular prejudices in rendering decisions. After the burning of his house and library in 1780, during anti-Catholic riots, which involved mobs of 50,000 and the invasion of Parliament itself, Mansfield so fairly conducted the treason trial of the leader, Lord George Gordon, that an acquittal resulted. In another case involving the prosecution of the journalist John Wilkes, who had published works that were declared seditious libel by the House of Commons, Mansfield rose above both popular clamour and royal pressure by careful technical work on precedents. His investigations showed that the crown's case contained legal flaws, and he felt himself forced to discharge an agitator because due process so required. A widespread legendary view that Mansfield abolished slavery in England with one judicial decision, while it took a civil war in the United States, is unfounded. As a property-minded man of commerce, Mansfield sought, with all of his high tactical powers, to avoid any slavery issue. Even his judgment in the so-called Somersett case (1772), involving the slave James Somersett, who was bought in Virginia and attempted to run away after arriving in London, decided only that an escaping slave could not be forcibly removed from England for retributive punishment in a colony.

Mansfield's permanent stamp upon Anglo-American law lies in commercial law. When he mounted the bench, at the start of the Seven Years' War that was to fasten Britain's grasp upon America, India, and international trade, English law was land-centred and landbound in outlook and entrenched in professional tradition. Reform was imperative. Mansfield's vision and ambition reached beyond the continental model of a special body of rules for commerce and banking. He sought to make the international law of commerce not a separate branch but an integral part of the general law of England, both common law and equity, using the leverage thus gained to pry loose from feudalism whole blocks of other rules that had little or no direct commercial bearing. An important part of this brilliant venture succeeded. In the area of bills of exchange (drafts), promissory notes, and the then still novel bank check, Mansfield, following standard international practice, shaped the law in sweeping judgments, each typically canvassing the whole relevant situation and its reasons. But Mansfield also established a new area of jurisprudence. Marine insurance, then a new industry, was centred in London and was a weapon of competition and cold war. Mansfield did not build here on models; he created the entire discipline.

He was not always successful. In 1765 he ruled that a merchant's or banker's confirmed credit, or promise to accept drafts drawn from abroad, was enforceable "without consideration"--i.e., without any bargained-for return. This decision was viewed as a flat attack on the whole legal doctrine of "consideration," and that doctrine was reaffirmed in its entirety by the House of Lords. He suffered a second defeat in his effort to make documents transferring land interpretable by "plain intention," so that such intention could not be frustrated by technical rules giving unmeant effect to words. His decision in this area was reversed in 1772 (one of only six reversals during his 32 years of active service). But he triumphed in his expansion of the idea that a man should turn back or turn over any value received by mistake or wrongdoing or under other circumstances making it inequitable for him to retain it. The remedy he devised was a fictitious assumption of a "promise" to pay over (in modern times the fiction was discontinued and replaced by the term "restitution"). Three times during his career Mansfield held positions as a member of the Cabinet, entrusting the great seal of his office to a committee, so that he could retain the chief justiceship regardless of changes in administration but still exert political power. In 1783 he declined Cabinet office, preferring to serve as speaker of the House of Lords. He resigned as chief justice in 1788.