The son of the 1st Earl of Halifax, he was styled Viscount Sunbury until succeeding his father as 2nd Earl of Halifax in 1739. Educated at Eton College and at Trinity College, Cambridge, he was married in 1741 to Anne Richards (died 1753), who had inherited a great fortune from Sir Thomas Dunk, whose name Halifax took. After having been an official in the household of Frederick, Prince of Wales, Lord Halifax was made Master of the Buckhounds, and in 1748 he became President of the Board of Trade. While filling this position he helped to found Halifax, the capital of Nova Scotia, which was named after him, and he helped foster trade, especially with North America.
About this time he attempted, unsuccessfully, to become a Secretary of State, but was only allowed to enter the Cabinet in 1757. In March 1761 Halifax was appointed Lord Lieutenant of Ireland, and during part of the time which he held this office he was also First Lord of the Admiralty. He became Secretary of State for the Northern Department under Lord Bute in October 1762, switching to the Southern Department in 1763 and was one of the three ministers to whom King George III entrusted the direction of affairs during the premiership of George Grenville. In 1762, in search of evidence of sedition, he authorised a raid on the home of John Entick, declared unlawful in the case of Entick v. Carrington. In 1763, he signed the general warrant for the "authors, printers and publishers" of The North Briton number 45, under which John Wilkes and 48 others were arrested, and for which, six years later, the courts of law made Halifax pay damages. He was also mainly responsible for the exclusion of the name of the King's mother, Augusta, Princess of Wales, from the Regency Bill of 1765.
Together with his colleagues, Lord Halifax left office in July 1765, returning to the Cabinet as Lord Privy Seal under his nephew, Lord North, in January 1770. He had just been restored to his former position of Secretary of State when he died. Halifax, who was Lord-Lieutenant of Northamptonshire and a Lieutenant-General in the Army, was very extravagant. He left no children, and his titles became extinct on his death. Lord Orford speaks slightingly of Halifax, and says he and his mistress, Mary Anne Faulkner, had sold every employment in his gift.