Onciul was born in Straja, at the time in the Duchy of Bukovina, Austrian Empire, now in Suceava County, Romania.
Together with Ioan Bogdan, Onciul founded a school-of-thought in Romanian historiography that approached history critically.
Much of his work dealt with and documented the formation of the early Romanian feudal states.
[1] The Romanian Academy established an historiography prize in his name, known as the "Dimitrie Onciul Award".
[2] There are streets named after him in Sector 2 of Bucharest, and in the cities of Cluj-Napoca, Pitești, and Suceava.