Early years and education
Jeffreys was born 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.
His brothers were people of note. Thomas, later Sir Thomas (knighted in 1686), was English Consul in Spain and a Knight of Alcántara. William was vicar of Holt, near Wrexham, from 1668 to 1675. His younger brother, James, made a good ecclesiastical career, becoming Vice-Dean of Canterbury in 1685.
George was educated at Shrewsbury School from 1652 to 1659, his grandfather's old school, where he was periodically tested by Philip Henry, a friend of his mother. He attended St Paul's School, London, from 1659 to 1661 and Westminster School, London, from 1661 to 1662. He became an undergraduate at Trinity College, Cambridge, in 1662, leaving after one year without graduating, and entering the Inner Temple for law in 1663.