William was born in 1728 and was baptised on 20 October 1728 at Ashton under Hill, Gloucestershire, England.
[2] His younger sister was Jacomina Bellenden, the wife of Thomas Orby Hunter, MP for Winchelsea, of Waverley Abbey in Surrey, in 1749.
[3] His father was the third son of John Ker (the third surviving son of William Ker, 2nd Earl of Roxburghe) and Lady Mary Moore (the second daughter of Henry Moore, 1st Earl of Drogheda).
[6] In 1804, upon the death of another unmarried and childless cousin John Ker, 3rd Duke of Roxburghe, the titles Earl Ker and Baron Ker, which had been created for his uncle in 1722 in the Peerage of Great Britain, became extinct and seventy-five-year old William succeeded to the dukedom, all of its other subsidiary titles,[1] and the family seat, Floors Castle in Roxburghshire on the banks of the River Tweed in south-east Scotland.
Mary was the daughter of Captain Benjamin Bechinne RN and Susanna (née Smith) Bechinne (sister of Sir John Smith, 1st Baronet).