She was unhappily married to John de Warenne, 7th Earl of Surrey. In 1345, Joan became the regent of Bar for her great-nephew Robert. Despite his claims, a divorce was never granted.
In 1345, Joan was invited by Philip VI of France to act as regent of the County of Bar.
In 1353, she returned to England. When John II of France was captured and imprisoned in London, she was allowed to visit him and is said, however unlikely the notion, to have become his mistress, at 56 years old.
Joan died in 1361 in London. In conspiracy theories, such as the one promoted in The Holy Blood and the Holy Grail, Joan of Bar has been alleged to be the fifth Grand Master of the Priory of Sion.