Caulfeild, William (1624?–1671), 5th Baron and 1st Viscount Charlemont , was third son among ten children of Sir William Caulfeild, 2nd baron, and his wife Mary, daughter of Sir John King (qv) (d. 1636), and was thus a younger brother of Toby Caulfeild (qv) (d. 1642). He fought on the parliamentary side in the English civil war and was taken prisoner at the battle of Newbury in 1644, the year he succeeded his brother Robert in the title of Baron Charlemont and inherited the family's large estates. The following year he was commissioned as a captain in the New Model Army. In Ireland in 1649 he was captured as he tried to join the parliamentary forces in Cork, though he was later freed in exchange for a brother of the marquess of Ormond (qv). He afterwards joined Sir Charles Coote's (qv) regiment. He was unable to gain possession of the family estates until after the defeat of Sir Phelim O'Neill (qv), who was blamed for the murder of Toby Caulfeild. O'Neill was betrayed, and Charlemont captured him on an island in Co. Tyrone in February 1653. He remained with Coote's regiment until its disbandment in 1655.
In the later 1650s he continued as commander of the family fort of Charlemont. In late 1659 or early 1660 he joined the council of officers formed in the aftermath of the December coup at Dublin castle; he subsequently signed the army declaration for a free parliament in February 1660, and represented Lifford, Co. Donegal, in the general convention in Dublin which met in March. After the restoration of the monarchy he was made a privy councillor, and confirmed as governor of the fort of Charlemont for life; however he sold this office to the crown for £3,500 in 1664, and in 1665 was created Viscount Charlemont. In 1670 he protected Oliver Plunkett (qv), the catholic archbishop of Armagh, from harassment, even to the extent of offering Plunkett his house for celebrating mass and performing confirmations.
Charlemont died in April 1671 and was buried in Armagh cathedral on 25 May. He married (1653) Sarah, daughter of Charles Moore (qv) (d. 1643), 2nd Viscount Drogheda; she was a granddaughter of Adam Loftus (qv) (d. 1605). They had four sons, and probably other children.