When Louis in 817 divided the Empire between his sons, Lothar was crowned joint emperor at Aix-la-Chapelle (Aachen) and given a certain superiority over his brothers. In 821 he married Irmengarde (d. 851), daughter of Hugo (Hugues), count of Tours; in 822 undertook the government of Italy; and, on the April 5, 823, was crowned emperor by Pope Paschal I at Rome, Italy.
In November 824 he promulgated a statute concerning the relations of pope and emperor which reserved the supreme power to the secular potentate, and he afterwards issued various ordinances for the good government of Italy.
On his return to his father's court his step-mother Judith won his consent to her plan for securing a kingdom for her son Charles the Bald, a scheme which was carried out in 829. Lothar, however, soon changed his attitude, and spent the succeeding decade in constant strife over the division of the Empire with his father. He was alternately master of the Empire, and banished and confined to Italy; at one time taking up arms in alliance with his brothers and at another fighting against them; whilst the bounds of his appointed kingdom were in turn extended and reduced.
When Louis I was dying in 840, he sent the imperial insignia to Lothar, who, disregarding the various partitions, claimed the whole of the Empire. Negotiations with his brother Louis the German and his half-brother Charles the Bald, both of whom armed to resist this claim, were followed by an alliance of the younger brothers against Lothar. A decisive battle was fought at Fontenay on June 25, 841, when, in spite of his personal gallantry, Lothar was defeated and fled to Aix. With fresh troops he entered upon a war of plunder, but the forces of his brothers were too strong for him, and taking with him such treasure as he could collect, he abandoned to them his capital.
Efforts to make peace were begun, and in June 842 the brothers met on an island in the Saone, and agreed to an arrangement which developed, after much difficulty and delay, into the Treaty of Verdun signed in August 843. By this Lothair received Italy and the imperial title, together with a stretch of land between the North and Mediterranean Seas lying along the valleys of the Rhine and the Rhone. He soon abandoned Italy to his eldest son, Louis, and remained in his new kingdom, engaged in alternate quarrels and reconciliations with his brothers, and in futile efforts to defend his lands from the attacks of the Northmen (as Vikings were known in Frankish writings) and the Saracens.
In 855 he became seriously ill, and despairing of recovery renounced the throne, divided his lands between his three sons, and on September 23, entered the monastery of Prüm (Pruem), where he died six days later. He was buried at Prüm, where his remains were found in 1860.
His kingdom was divided among his three sons - the eldest, Louis II, received Italy and the title of Emperor; the second, Lothar II, received Lotharingia, while the youngest, Charles, received Burgundy.
See "Annales Fuldenses"; Nithard, "Historiarum Libri," both in the Monumenta Germaniae historica. Scriptores, Bände i. and ii. (Hanover and Berlin, 1826 fol.); E. Mühlbacher, Die Regesten des Kaiserreichs unter den Karolingern (Innsbruck, 1881); E. Dummler, Geschichte des ostfränkischen Reichs (Leipzig, 1887-1888); B. Simson, Jahrbücher des deutschen Reiches unter Ludwig dem Frommen (Leipzig, 1874-1876).