Lucy Walter or Lucy Barlow (c. 1630 – 1658) was the mistress of the English king Charles II and mother of the Duke of Monmouth. She is believed to have been born in 1630 or a little later at Roch Castle near Haverfordwest, Wales into a family of middling gentry.
Lucy Walter was the daughter of William Walter of Haverfordwest and wife, Elizabeth Protheroe, a Welsh noblewoman descended from the second marriage of Thomas Howard, 2nd Duke of Norfolk. The Walters were a Welsh family of good standing, who declared for the king during the Civil War. Roch Castle was captured and burned by the parliamentary forces in 1644, and Lucy Walter found shelter first in London and then at the Hague.
She entered the fringes of London society through family connections, and at the age of seventeen was the mistress of Algernon Sidney, a Roundhead officer related to the Earl of Leicester. In the Netherlands she met his younger brother, a Royalist exile, Robert Sidney, with whom she began an affair. It was through Robert that she met Charles II.
There is little evidence to support the story that she was the first wife of Charles II, and it is certain that he was not her first lover in royal circles. The intimacy between him and this "brown, beautiful, bold but insipid creature," as John Evelyn called her, who chose to be known as Mrs Barlow (Barlo), lasted with intervals until the autumn of 1651, and Charles claimed the paternity of a child born in 1649 whom he subsequently created Duke of Monmouth.
After her relationship with Charles II ended for unknown reasons, she led a poor and dissolute life, which possibly resulted in her premature death at Paris in September/October 1658. The cause of death is unknown.