Politician and noble. Ker acceded to the Earldom of Roxburghe, following the death of his elder brother Robert, the 4th Earl (d.1682). He was Secretary of State for Scotland (1704-5) and was rewarded for being an enthusiastic promoter of the Act of Union by being created Duke of Roxburghe by Queen Anne in 1707. Ker was also a Knight of the Garter.
He commissioned William Adam to build the grand Floors Castle (1725) as a seat befitting his stature.