After attending school in his hometown, Lehmann started studying at the University of Göttingen.
Author of a dissertation on Franciscus Modius and a Habilitationsschrift on Johannes Sichardus, he made numerous contributions to the Sitzungsberichte der bayerischen Akademie.
He also authored Pseudo-Antike Literatur des Mittelalters (1927) and published Mittelalterliche Bibliothekskataloge Deutschlands und der Schweiz.
Lehmann assisted Max Manitius in the preparation of the third volume of the Geschichte der lateinischen Literatur des Mittelalters.
A Festschrift entitled Liber Floridus, in honor of his sixty-fifth birthday, was published in 1950.