Bertie retired from active service in 1809 upon inheriting the title of Earl of Lindsey from his third cousin on 8 February 1809. The earldom had been held by Robert Bertie, 1st Marquess of Lindsey from 1706 until 1715 when he was he was created the 1st Duke of Ancaster and Kesteven. The Dukes of Ancaster and Kesteven held the earldom until the dukedom became extinct on the death of Brownlow Bertie, 5th Duke of Ancaster and Kesteven in 1809, and the earldom passed to Bertie.