After the marriage of William and Mary in May 1641, she followed her husband, Lord Stanhope, to Holland as the governess to the Princess Royal.

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 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.