John Campbell served as a Field Marshall in the Royal Army, and represented Glascow in Parliament from 1744-61, and Dover from 1765-66. On 22 December 1766, he was named Baron Sundridge, of Coomb Bank, Kent (with the remainder to his two brothers). He succeeded his father as Duke in 1770. On 3 March 1759, he married Elizabeth [d. 1790], widow of the 6th Duke of Hamilton and daughter of John Gunning of Castle Coote, county Roscommon, Ireland. Named a Baroness in her own right in 1776, Campbell's wife was known as one of the greatest beauties of the court of George III. The couple had three sons and two daughters; sons George William and John Douglas succeeded their father as 6th and 7th Dukes, respectively.