Rupert was a brilliant cavalry officer, having gained some experience in the Thirty Years War. In 1642, King Charles appointed him to lead the Royalist cavalry, and their early successes are largely attributable to Rupert. His dashing reputation earned him the nickname of the "Mad Cavalier". He is reputed to have taken a large dog, "Boy", into battle with him on several occasions.
As Parliament's army gained the ascendant, Rupert's skills were not enough to save the Royalists, and when, in 1645, he surrendered Bristol, his uncle rejected him, forgetful of his earlier sterling service. Rupert was court-martialled at his own request and cleared of any blame, but he never enjoyed the king's favour as he had previously done, and in 1646 he left England.
By 1648 Rupert was fighting with the French against Oliver Cromwell's England. However, following a naval defeat by Admiral Robert Blake, he took refuge in the West Indies, where he followed the life of a buccaneer, preying on English shipping. Following the Restoration of the monarchy, he returned to the service of England. After his retirement from the military in around 1670, he engaged in scientific research, and has sometimes been credited with the invention of the mezzotint, as well as a form of gunpowder and an alloy named "Prince's metal" in his honour.