George Fitzroy, 1st Duke of Northumberland (Oxford, December 28, 1665 - Epsom, June 28, 1716) was the third and youngest illegitimate son of King Charles II. His mother was Barbara Villiers, Countess of Castlemaine (also known as Barbara Villiers, Duchess of Cleveland). On October 1, 1674, he was created Earl of Northumberland, Baron of Pontefract (Yorkshire) and Viscount Falmouth (Cornwall). On April 6, 1683, he was created Duke of Northumberland.

The first Duke of Northumberland was born at Merton College, Oxford, on December 28, 1665. In 1682, he was employed on secret service in Venice. Upon his return to England in 1684, he was elected (January 10, 1684) and installed (April 8, 1684) Knight of the Garter. That summer, he served as a volunteer on the side of the French at the Siege of Luxembourg. In 1687, the Duke of Northumberland commanded the second troop of horse guards. A year later, he was appointed a lord of His Majesty's bedchamber. In 1701, he was appointed Constable of Windsor Castle and Lord-Lieutenant of Surrey. In 1712, he would become Lord-Lieutenant of Berkshire as well. In 1703, he succeeded the Earl of Oxford as Colonel of the Royal Regiment of Horse. Seven years later, on January 10, 1710, he became Lieutenant-General. On April 7, 1713, he was sworn of the Privy Council and Chief Butler of England.

In March 1686, the Duke of Northumberland married Catherine Wheatley, the daughter of poulterer Robert Wheatley of Bracknell, Berkshire. Catherine Wheatley was the widow of Thomas Lucy of Charlecote, a captain in the Royal Horse Guards. Soon after the marriage, the Duke of Northumberland and his brother, Henry FitzRoy, 1st Duke of Grafton, allegedly attempted to privately convey her abroad to an English convent in Ghent, Belgium. After the death of Catherine in 1714, the Duke of Northumberland remarried to Mary Dutton, the sister of Captain Mark Dutton.

The Duke suddenly died at Epsom on June 28, 1716. He had no legitimate offspring. Mary died at Frogmore House in 1738.