He was trained as a lawyer and became sub-commissioner of public records in Dublin Castle. He was an active member of the Royal Irish Academy and collected and rescued many examples of Irish traditional music.
In 1855, shortly after its foundation, he became librarian of Queen's College, Galway (now the National University of Ireland, Galway). The university library now bears his name.
Hardiman is best remembered for his History of the Town and County of Galway (1820) and Irish Minstrelsy (1831), one of the first published collections of Irish poetry and songs.