After the marriage of William and Mary in May 1641, she followed her husband to Holland as the governess to the Princess Royal. As the princess came to age, Lady Stanhope grew to become her confidante and adviser. During the English Civil War, Lady Stanhope sided with Charles I and his heir, King Charles II; she is said to have supplied them both politically and financially, and to have been involved in much of the royalist plots of that decade.
After the death of Heenvliet in 1660, Charles II made her the Countess of Chesterfield in recognition of both her service and her friendship. She remained in princess Mary's service until the latter's death from illness on 24 December 1660. She then passed into the service of Anne Hyde, Duchess of York, and in 1662 to Queen Catherine of Braganza, wife of Charles II.
In 1662, Lady Catherine married her friend Daniel O'Neill (d.1664), Postmaster General, another one of the King's men during the civil war. Upon his death in 1664, she increased her by then already considerable wealth by inheriting O'Neill's office of Postmaster General.[1]
Secondly in early 1641, after being courted by several suitors, she married the Dutchman Jehan van der Kerckhove, Lord of Heenvliet (d.1660), one of the diplomats involved in negotiating the marriage between William II, Prince of Orange and Mary, the Princess Royal, daughter of King Charles I, future parents of King William III of England.