Cagnes was the town where in 1309 he established a stronghold, today known as the Château Grimaldi.

He was the eldest of the three sons of Lanfranco Grimaldi, French Vicar of Provence, by his wife, Aurelia del Carretto (who later remarried her husband's cousin, François Grimaldi).

In 1297, Rainier joined his stepfather (François) and a group of men to take the castle on the Rock of Monaco.

[citation needed] 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).

[citation needed] Rainier held the citadel of Monaco for four years before departing on April 10, 1301.