From 1883 to 1884 he was for a short time an "unskilled worker" (at that time a common term for scientific staff) at the Monumenta Germaniae Historica (MGH), where he supported Ernst Dümmler in the edition of the second MGH Poetae volume.
[2] In 1884 he started working as a teacher at the Noldensian Higher School for Girls in Dresden, which gave him enough time for further medieval research, with which he soon made a living.
Still in 1884 he published a critical edition of an anonymous 9th century geographical writing, De situ orbis.
In 1889 Manitius published a complete representation of the 10th and 11th centuries under the title German History under the Saxon and Salian Emperors (911-1125).
The Munich philologist and MGH central director Ludwig Traube († 1907) had originally been engaged to prepare this volume, but he had allowed himself to be released from his contract and instead recommended Manitius to private scholars.