On the death of his brother Charles in 863 he added some lands south of the Jura to this inheritance, but, except for a few feeble expeditions against the Danish pirates, he seems to have done little for its government or its defence.
The reign was chiefly occupied by efforts on the part of Lothair to obtain a divorce from his wife Teutberga, a sister of Hucbert, abbot of St Maurice (d. 864); and his relations with his uncles Charles the Bald and Louis the German, were influenced by his desire to obtain their support to this plan. Although quarrels and reconciliations between the three kings followed each other in quick succession, in general it may be said that Louis favoured the divorce, and Charles opposed it, while neither lost sight of the fact that Lothair had no sons to inherit his lands. Lothair, whose desire for the divorce was prompted by his affection for a certain Waldrada, put away Teutberga; but Hucbert took up arms on her behalf, and after she had submitted successfully to the ordeal of water, Lothair was compelled to restore her in 858. Still pursuing his purpose, he won the support of his brother, the emperor Louis II, by a cession of lands, and obtained the consent of the local clergy to the divorce and to his marriage with Waldrada, which took place in 862.
A synod of Frankish bishops met at Metz in 863 and confirmed this decision, but Teutberga fled to the court of Charles the Bald, and Pope Nicholas I declared against the decision of the synod. An attack on Rome by the emperor was without result, and in 865 Lothair, convinced that Louis and Charles at their recent meeting had discussed the partition of his kingdom, and threatened with excommunication, again took back his wife. Teutberga, however, either from inclination or compulsion, now expressed her desire for a divorce, and Lothair went to Italy to obtain the assent of the new pope Adrian II. Placing a favourable interpretation upon the words of the pope, he had set out on the return journey, when he was seized with fever and died at Piacenza on the August 8, 869. He left, by Waldrada, a son Hugo who was declared illegitimate, and his kingdom was divided between his uncles Charles the Bald and Louis the German.