Gisulf died in battle; Grimoald and his brother Radoald escaped to Beneventum, where they were adopted by Duke Arechis, a distant kinsman.

[3] As duke, he successfully defended the Sanctuary of Monte Sant'Angelo on Mount Gargano from "Greeks", possibly from Naples, who had come to plunder.

In 662 the brothers fell out, and Godepert sent Garipald, Duke of Turin to persuade Grimoald to assist in a war against Perctarit.

[6] He saved the northeast of Italy by defeating the Slav tribes and maintained internal order by suppressing the baronial revolts and autonomy of the duchies of Friuli and of Spoleto, where he installed Thrasimund.

In his religion, he remained nominally Arian (though according to Vita Sancti Barbati both he and his son Romuald still practiced the ancient Pagan rites of both Benevento and the Longobard nation) despite his marriage to a Catholic.

However, he perceived Saint Michael—whose cult was spreading strongly from Monte Gargano—as the warrior-protector of the Lombard nation, replacing Wodan (Odin) due to their similar narratives at the time.

Grimoald was buried in the Basilica of St. Ambrose in Pavia His son Garibald was elected to succeed him and was then deposed by the once exiled Perctarit in three months time.