Mommsen studied jurisprudence in Kiel from 1838 to 1843, then he went to France and Italy to study classical history. A professor of law at the University of Leipzig, he was involved in the 1848 revolution and had to resign in 1850.
He held posts at the University of Zürich and the University of Breslau. In 1858 he was professor of Ancient History at the University of Berlin, then he was named permanent secretary of the Prussian Academy of Arts and Sciences. He was later elected a member of the parliament of Prussia as a National Liberal (later as a Liberal).
Mommsen published hundreds of works - a 1905 bibliography lists over 1,000 items - and effectively gave a new order to the study of Roman history. He pioneered epigraphy, the study of inscriptions on stone and wood. His main work was the unfinished History of Rome, but today his most relevant work is perhaps the Corpus Inscriptionum Latinarum, a collection of Roman inscriptions he contributed to for the Berlin Academy. Other works regarded Roman coinage and Roman constitutional and criminal law. He eidted several volumes of the Monumenta Germaniae Historica.