Pepin was baptized at Rome, 12 Apr 781, by Pope Adrian I. Apparently by a daughter of Duke Bernard, who was the younger brother of Pepin the Short. Pepin's name was Carloman at birth. However, when his brother Pepin the Hunchback betrayed their father, the royal name Pepin passed to him. After his father's conquest of the Lombards, he was crowned in 781 by Pope Hadrian I with the Iron Crown of Lombardy.

Pepin expanded the Frankish empire and in 791 he marched his Lombard army into the Drava Valley where they ravaged Pannonia. At the same time his father, Charlemagne, marched with his army along the Danube into Avar territory. Charlemange had to leave to squelch a Saxon revolt in 792. Pepin and Duke Eric of Friuli continued their campaign against the Avars' ring-shaped strongholds. It was taken twice and the spoils were sent to King Charlemagne. He divided it with his followers and also sent some to King Offa of Mercia and other foreign rulers.

Pepin's army was unsuccessful at the seige of Venice in 810. It lasted six months. Pepin's army became ravaged by the disease of the swamps which forced their withdrawal. Pepein died a few months later on 8 Jul 810 in Milan, Milan, Italy.