The Walters were a Welsh family of good standing, who declared for the king during the Civil War. Roch Castle having been captured and burned by the parliamentary forces in 1644, Lucy Walter found shelter first in London and then at the Hague. There, in 1648, she met the future king, possibly renewing an earlier acquaintance.
There is little reason for believing the story that she was his first mistress; it is certain that he was not her first lover. The intimacy between him and this "brown, beautiful, bold but insipid creature," as John Evelyn calls her, who chose to be known as Mrs Barlow (Barlo) lasted with intervals till the autumn of 1651, and Charles claimed the paternity of a child born in 1649, whom he subsequently created duke of Monmouth. A daughter, Mary (b. 1651), of whom the reputed father was Henry Bennet, earl of Arlington, married William Sarsfield.