The eldest of the three sons of Lanfranco Grimaldi, French Vicar of Provence, by his wife, Aurelia del Carretto (who later was remarried to her late husband's nephew, François Grimaldi), Rainier joined his stepfather and a group of men to take the castle on the Rock of Monaco; the event is commemorated on the Monegasque coat of arms, where the supporters are two monks armed with swords (because François dressed as monk and opened the gates of Monaco's castle). Rainier held the citadel of Monaco for four years before departing on April 10, 1301. In 1304 he was appointed Admiral of France after winning the Battle of Zierikzee.