Son of Arechi II and Adelperga, after the conquest of the Lombard Kingdom by the Frankish king Carlo Magno (774) he was sent by his father to the Carolingian court as a hostage, obtaining in exchange the autonomy of the duchy which in the meantime he had raised to the rank of principality. The young man was thus taken to Aachen and educated in obedience to the Franks; but in Salerno (where the Lombard court had moved from Benevento) between July and August 787, both Arechi II and his eldest son Romualdo died in quick succession.

Grimoaldo obtained from Carlo Magno the authorization to re-enter the principality and assume the crown, however committing himself to mint coins and to issue documents exclusively in Carlo's name and on the condition that he demolish the imposing defensive works built by his father and that he help the Franks to fight Adelchi, son of the last Lombard king Desiderius.

Upon returning home, Grimoaldo obeyed Carlo Magno, but once Adelchi was defeated, as reported by the "Chronicon Salernitanum", he decided to build even more impressive and vast fortifications in Salerno than the previous ones, thus breaking the alliance between Benevento and the Frankish king.