The name comes from a Latinization of Morganuc, "South Glamorgan in the Domesday Book", the county where it was discovered by Walter Georg Kühne, so it means "Glamorgan tooth". It has acquired the nickname Morgie in the Smithsonian National Museum of Natural History.
Morganucodon was a small, plantigrade animal. The tail was moderately long. According to Kemp (2005) "the skull was 2-3 cm in length and a presacral body length of about 10 cm [4 inches]. In general appearance it would have looked like a shrew or mouse". There is evidence that it had specialized glands used for grooming, which may indicate that, like more advanced mammaliaformes, it had fur.
The diet appears to have been insects and other small animals, again much like a modern shrew. Like most modern mammal insectivores, it grew fairly quickly to adult size. Unlike its therapsid ancestors, Morganucodon likely lived a rather short life, similar to those of most small mammals today. Its eggs were probably small and leathery, a condition still found in monotremes.
The teeth grew in mammalian fashion, with deciduous teeth being replaced by permanent teeth that were retained throughout the rest of the animal's life. Unlike the situation in most later mammals, however, the upper and lower molars did not occlude properly when they first met; as they wore against each other, however, their shapes were modified by wear to produce a precise fit.
Morganucodon is the type genus for the order Morganucodonta, a group of generally similar mammals or pre-mammals from Late Triassic to Middle Jurassic time of the close relatives. All were small and mainly insectivores. Of the small bodied relatives, Morganucodon is the best preserved and best understood find.
There is currently controversy about whether or not to classify Morganucodon as a mammal or as a non-mammalian mammaliaform. Some researchers limit the term "mammal" to the crown group mammals, which would not include Morganucodon and its relatives. Others, however, define "mammals", as a group, by the possession of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which has replaced the primitive one between the articular and quadrate bones in all modern mammalian groups. Under this definition, Morganucodon would be a mammal. Nevertheless, its lower jaw retains some of the bones found in its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary. Furthermore, the primitive reptile-like jaw joint between the articular and quadrate bones, which in modern mammals has moved into the middle ear and become part of the ear ossicles as malleus and incus, is still to be found in Morganucodon.