Grimoald was born in 610 A.D. as the heir of Duke Gisulf II of Friuli and the Bavarian Princess Ramhilde, daughter of Duke Garibald I of Bavaria. From 641 to 642, he and his brother Radoald served as regents to Duke Aiulf I, their adoptive brother. In 647, Grimoald succeeded Radoald as Duke of Benevento. In 662, he assassinated King Godepert and proclaimed himself King of the Lombards. He married Princess Theodota, daughter of King Aripert I.