Jeffreys was born on 15 May 1645 at the family estate of Acton Hall, in Wrexham, in North Wales, the sixth son of John and Margaret Jeffreys. His grandfather, John Jeffreys (died 1622), had been Chief Justice of the Anglesey circuit of the Great Sessions. His father, also John Jeffreys (1608–1691), was a Royalist during the English Civil War, but was reconciled to the Commonwealth and served as High Sheriff of Denbighshire in 1655.
He was educated at Shrewsbury School, at St. Paul's School in London, at Westminster, and at Trinity College, Cambridge.
In 1663 he became a student in the Inner Temple, where he remained until 1668, when he was admitted to the bar.
He embarked on a legal career in 1668, becoming a Common Serjeant of London in 1671. He was aiming for the post of Recorder of London, but was passed over for this in 1676 in favour of William Dolben. He turned instead to the Court and became Solicitor General to the Duke of York and of Albany (later King James II & VII), the younger brother of Charles II. Despite his Protestant upbringing, he found favour under the Roman Catholic Duke. In September 1677 he was appointed solicitor general to James, Duke of York, who later became King James II.
He took a prominent part in the scandalous trials of Catholics implicated in the alleged Popish Plot of 1678, and in November 1681 he was knighted at Whitehall.
Holding close to favor and the court party, he was raised to the peerage by James II in 1685.
In the same year he carried through his so-called Bloody Assizes, the victims of which were prisoners taken in suppressing the Monmouth Rebellion, an uprising headed by James, Duke of Monmouth, whose aspirations to the throne led to his capture and beheading in 1685.
More than 300 persons were executed, many more were imprisoned and fined, and hundreds more were sent to the American colonies as bondsmen.
As a reward, Jeffreys was appointed Lord Chancellor in 1685, and as such he vigorously upheld the extreme claims of prerogative made by James II. In 1687 he was appointed Lord Lieutenant of Shropshire and of Buckinghamshire. Jeffreys' historical notoriety comes from his actions in 1685, after Monmouth's Rebellion. Jeffreys was sent to the West Country in the autumn of 1685 to conduct the trials of captured rebels. Estimates of the numbers executed for treason have been given as high as 700, however, a more likely figure is between 160 and 170 of 1381 defendants found guilty of treason. Although Jeffreys has been traditionally accused of vindictiveness and harsh sentencing, none of the convictions have been considered improper, except for that of Alice Lisle tried at Winchester. Furthermore, as the law of the time required a sentence of death for treason, Jeffreys was required to impose it, leaving the king the option of commuting sentence under the prerogative of mercy. Arguably, it was James II's refusal to use the prerogative as much as was customary for the time, rather than Jeffreys' actions that made the government's reprisals so savage. During the Glorious Revolution, when James II fled the country, Jeffreys stayed in London until the last moment, being the only high legal authority in James's abandoned kingdom to perform political duties. When William III's troops approached London, Jeffreys tried to flee and follow the King abroad. He was captured in a public house in Wapping, now named The Town of Ramsgate. Reputedly he was disguised as a sailor, and was recognized by a surviving judicial victim, who claimed he could never forget Jeffreys' countenance, although his ferocious eyebrows had been shaven. Jeffreys was terrified of the public when dragged to the Lord Mayor and then to prison "for his own safety". He begged his captors for protection from the mob, who intended "to show him that same mercy he had ever shown to others".
He died of kidney disease (probably pyelonephritis) while in custody in the Tower of London on 18 April 1689.
He was a member of President of the Ecclesiastical Commission.
His legal ability was undoubtedly high, and he was definitely good in all cases that required him to rule on questions of law, but not of loyalty. Some say he was a personally vengeful man. He suffered from a painful kidney disease that may well have affected his unbridled temper and added to this reputation, and his doctors apparently recommended alcohol to dull the pain, which may have explained his often shocking conduct in court.
In 1667, he married Sarah Neesham or Needham, by whom he had seven children; she died in 1678. She was the daughter of the impoverished vicar of Stoke d'Abernon, Thomas Neesham. He married secondly in 1679, Anne, daughter of Sir Thomas Bloodworth, who was Lord Mayor of London 1665-6; she was the widow of Sir John Jones of Fonmon Castle, Glamorgan. Being only 29 at the time of her second marriage, she was described as a 'brisk young widow' and there were some rumours about her. She was said to have a formidable temper: Jeffreys' family went in awe of her, and it was said she was the only person he was afraid of; a popular ballad joked that while St. George had killed a dragon and thus saved a maiden in distress, Sir George had missed the maiden and married the dragon by mistake.
