[3] His paternal grandfather was Sir Walter Ker of Cessford (died c. 1584), who fought against Mary, Queen of Scots, both at Carberry Hill and at Langside.
[6] In December 1590 he was involved in the assassination of William Kerr of Ancram who was ambushed on the stairs at the entry to his lodging by two of Robert's followers who shot him with a pistol called a "dag".
In August 1594 he performed in the tournament at the baptism of Prince Henry at Stirling Castle dressed as a Turkish knight.
In April 1601 Roxburghe invited James VI and his consort Anne of Denmark to the celebration of the marriage of his sister Elizabeth to Sir John Bellenden of Broughton at Floors Castle, but the king refused, suspecting a plot involving the Master of Gray.
On 18 September 1616, he was created Lord Ker of Cessford and Caverton, and Earl of Roxburghe with remainder to his heirs male.
[13] He offended decorum at court in April 1625, when the body of King James was brought from Theobalds it was noticed that the Earls of Morton and Roxburghe were not in attendance, but went to be "merry" at More Park with Lucy Russell, Countess of Bedford.
When the King attempted to arrest five members of the House of Commons in 1642, Ker kept the door of the chamber open[15] (see article on William Lenthall).