After his despotic and unpopular father retired, or was forced by him, to enter the monastery of San Massimo, he took over the reins of government completely.
At first, he continued the Byzantine alliance of his father and received the titles of patricius and protospatharius.
He also allied himself to Capua, then united to the Principality of Benevento, by marrying Atenulf I's daughter Gaitelgrima.
Landulf called in the assistance of Theobald of Spoleto, but the unscrupulousness of the latter broke down the alliance and, in the early 930s, Guaimar returned to the Byzantine fold, with much persuasion from the protospatharius Epiphanius.
In 940, at the urging of his wife, he accepted the exiled Landulf of Benevento and his sons, bestowing on them territories in Salerno.