Catherine Sedley, born in 1657, was the only child of Sir Charles Sedley, Bart. of Aylesford, Kent. She was for many years mistress to James, Duke of York, by whom she had Catherine, later Duchess of Buckingham and Normanby. On his accession as James II, the King confered on her the titles of Baroness of Darlington and Countess of Dorchester for life. In 1696 she married Sir David Colyear, 2nd Bart., afterwards Earl of Portmore (1703). Catherine died in 1717 leaving two sons, the second of whom became the 2nd Earl of Portmore.