He was born in Marne, Holstein as the second son of merchant Johann Anton Müllenhoff.

[1] He later studied under Gregor Wilhelm Nitzsch at the University of Kiel, then continued his education at Leipzig (1839, under Gottfried Hermann and Moriz Haupt) and then in Berlin (1839-1841), where his instructors included Karl Lachmann and Wilhelm Grimm.

Two of his well-known students in Berlin were Wilhelm Scherer and Elias von Steinmeyer.

[1][2] In 1863 he introduced a theory involving the continuity of written language dating from the era of Old High German.

[1] From around 1875, he resided at Schellingstraße 7 (Berlin-Tiergarten), a few years later, moving to a house on Lützowufer in the same district.