He entered the faculty at Halle in 1855, and started an historical Seminar. In 1858 he became professor extraordinary, in 1866 full professor. In 1875 he became a member of the revised committee directing the Monumenta Germaniae Historica, himself undertaking the direction of the section "Antiquitates," and in 1888 became president of the central board in Berlin. This was an official recognition of Dümmler's leading position among German historians.
In addition to numerous critical works and editions of texts, he published:
Piligrim von Passau und des Erzbistum Lorch (1854)
His great work was the Geschichte des ostfrankischen Reiches (Berlin, 1862-1865, Sn. 2 vols.; 2nd ed. 1887-1888, in 3 vols.). In conjunction with Wattenbach he completed the Monumenia Alcuiniana (Berlin, 1873), which had been begun by Philipp Jaffe, and with R. Köpke he wrote Kaiser Otto der Grosse (Leipzig, 1876).
He edited the first and second volumes of the "Poetae latini aevi Carolini" for the Monumenta Germaniae historica (Berlin, 1881-1884).