Her full name was Eleanor Gwyn and she was born in Hereford, probably of Welsh ancestry, her father being a Thomas Gwyn. Having first made a living selling oranges, she became an actress (not at that time a respectable profession) when she was fifteen. When she was 19 she became the king's mistress, having previously been the mistress of Lord Brockhurst. She had two children by Charles. The elder, Charles Beauclerk was created Duke of St Alban's.
She was known for her wit as well as her beauty. In 1681, when her carriage was booed by a throng that had mistaken her for another of Charles II's mistresses, the Catholic Louise de Keroualle, she responded "Pray, good people, be civil; I am the Protestant whore".
It is thought to have been Nell who persuaded the king to build the Royal Hospital, Chelsea in London for ex-servicemen. His dying words, in 1685, are sometimes said to have been, "Let not poor Nelly starve." She didn't.