Jephson, William (1615–58), soldier and politician, was the son and heir of Sir John Jephson of Froyle, Hampshire, England, and his wife Elizabeth, daughter of Sir Thomas Norreys of Mallow Castle, Co. Cork. Jephson was a true Anglo-Irishman, having estates and family connections in both England and Ireland, and his relatives and friends included John Pym and Viscount Saye and Sele, as well as Munster planters such as Sir Philip Perceval (qv), Sir William Fenton, and Sir William St Leger (qv). He married (1636) Alice, daughter of Sir John Denham of Borestall House, Buckinghamshire. Jephson's political career was divided between England and Ireland. Elected for Stockbridge in Hampshire for both the Short and Long Parliaments in 1640, Jephson became politically active only after the outbreak of the Irish rebellion in October 1641, when he used his connections to promote the cause of the Munster protestants at Westminster. In February 1642 he returned to Ireland, and defended Mallow castle against the catholic insurgents in the next few months. In the faction fighting between the 1st earl of Cork (qv) and Lord Inchiquin (qv), which hampered the war effort in the early 1640s, Jephson sided with Inchiquin, and he returned to England in 1643 as his agent. In April 1643 he attended Charles I in Oxford as part of an Irish delegation hoping to secure a second adventurers’ act, and the king's refusal prompted him to return to Westminster, where he became a leading critic of royal attempts to make peace with the Irish catholics. He worked closely with Sir John Clotworthy (qv) to discredit the cessation of arms eventually signed in September 1643. Jephson took the solemn league and covenant on 7 February 1644, and was appointed lieutenant-governor of Portsmouth on 14 May. When Lord Inchiquin finally lost patience with the cessation in July 1644, Jephson was on hand to broker his defection to parliament.
In May 1645 Jephson lost his position at Portsmouth under the self-denying ordinance, and was able to devote his attention to the situation in Munster. He served on the new committee of Irish affairs appointed in July, and raised a regiment of horse, but with the appointment of Viscount Lisle (qv) as parliament's lord lieutenant (January 1646) he became embroiled in the factionalism which came to dominate Irish business at Westminster, and soon found himself at loggerheads with Lisle's principal Munster ally, Lord Broghill (qv). He supported the presbyterian resurgence in the spring of 1647, and when Lisle and his friends were ousted from Munster in April, he succeeded Broghill as lieutenant-general of horse. With the eventual victory of Lisle's friends in the Independent faction at Westminster in the summer of 1647, Jephson's influence over Irish affairs rapidly diminished. He failed in his attempts to prevent Inchiquin from defecting back to the king in April 1648, and thereafter took little part in parliamentary business. He was excluded from the commons during Pride's purge on 6 December 1648.
Although Jephson kept on reasonably good terms with the commonwealth regime, he was not active during the early 1650s, partly because of his mounting financial difficulties, which forced him to sell his Hampshire estates. He returned to Ireland in July 1653, after the Cromwellian conquest of the island had been completed. In Ireland, Jephson secured his estates largely through the benevolence of Oliver Cromwell (qv) and his son Henry (qv). He also made his peace with Lord Broghill, and it was through the interest of the Boyle family that he was elected as MP for Cork and Youghal in the union parliament of 1654. At Westminster, Jephson was allied with the ‘court’ party, and worked with Broghill and Henry Cromwell in the house. He was again returned for Cork and Youghal in 1656, and in the second protectorate parliament supported Broghill's successful attempt to overthrow the militia bill on which the English major-generals depended. He was also one of the first MPs to raise the question of crowning Oliver Cromwell, and was a firm supporter of the offer of the crown made under the ‘Humble petition and advice’ in the spring of 1657, orchestrated by Broghill and other ‘courtiers’. Cromwell's refusal of the kingly title was a devastating blow to Jephson, and on 27 May 1657 he even proposed, bitterly, that the letters k-i-n-g should be expunged from the English alphabet, as they caused so much offence.
Jephson's reward for his loyalty to the house of Cromwell was his appointment, in July 1657, as ambassador extraordinary to the king of Sweden, Carl X Gustav. His mission was to encourage the Swedish king to make peace with the king of Denmark, and to enter an alliance with Britain that would form the basis for a pan-protestant league against Poland and Austria. The treaty between the Swedes and the Danes was concluded in February 1658. With this achieved, Jephson moved on to Berlin, where he failed to persuade the elector of Brandenburg to make peace with Sweden. Jephson's diplomatic endeavours were hampered by his continuing financial problems, and by ill health. On his return to England he did not travel further than the house of his wife's family in Buckinghamshire, where he drew up his will on 6 December 1658, in which he assigned his eldest son to the care of Henry Cromwell, now lord lieutenant of Ireland. Jephson died 11 December 1658, and was succeeded by his son, Sir John Jephson. His descendants remained at Mallow castle till 1911.