Appointed duke of Brittany in 826 by the Carolingian emperor Louis I the Pious, Nomenoë quelled a serious revolt in 837. When Louis died and war broke out between his sons in 840, Nomenoë was first a vassal to Charles the Bald, the youngest of them, but then he occupied Rennes, which Charles besieged in 843. After defeating Charles in 844 and 845, Nomenoë gained recognition as an independent ruler.
Nomenoë became a crowned king after he had removed from Breton sees the French bishops, who had refused to crown him in 849. His actions, however, led to a split between the Breton church and the Archbishop of Tours. Nomenoë widened the split further by establishing his own archbishopric at Dol, but it never received papal sanction. In 849–850 he ravaged Anjou and seized both Nantes and Rennes in order to augment his power as an independent ruler.