Though we're never explicitly told that Mablung was a Dúnadan, Faramir referred to his Rangers, including Mablung, as generally belonging to that people. That fact, and Mablung's possession of an Elvish name, make his identity as one of the Dúnedain relatively sure.

This apparently curious derivation comes from the fact that Mablung shared his name with an Elf of the First Age, Mablung of the Heavy Hand. That earlier Mablung in turn got his name from a famous incident that involved the dropping of the hand of Beren and the Silmaril it held - see the entry for Mablung of the Heavy Hand for all the details.