He was the eldest son of Charles the Bald and Ermentrude of Orléans. He succeeded his younger brother in Aquitaine in 866 and his father in West Francia in 877, though he was never crowned Emperor. In the French monarchial system, he is considered Louis II.

He was crowned on 8 December 877 by Hincmar, archbishop of Rheims, and was crowned a second time in September 878 by Pope John VIII at Troyes while the pope was attending a council there. The pope may even have offered the imperial crown, but it was declined. Louis the Stammerer was said to be physically weak and outlived his father by only two years. He had relatively little impact on politics. He was described "a simple and sweet man, a lover of peace, justice, and religion". In 878, he gave the counties of Barcelona, Girona, and Besalú to Wilfred the Hairy. His final act was to march against the Vikings who were then the scourge of Europe. He fell ill and died on 10 April or 9 April 879 not long after beginning his final campaign. On his death, his realms were divided between his two sons, Carloman and Louis.