The philologist is regarded as the preserver of verbal tradition, but as a philologist Mommsen was more than that: he was an artist, and he proved his artistry in his treatment of language. He disliked any incongruous mixture of prose styles and, in the Romische Geschichte and Romisches Staatsrecht (“Roman Constitutional Law”), he created two works, both of which attain exemplary unity of form and content yet demonstrate two different styles. Without being a creative poet, he used the means of poetry and enjoyed exercising his poetic talent. An excellent testimony to his abilities is the Liederbuch dreier Freunde (“Songbook of Three Friends”), which he published in 1843 together with his brother Tycho and the writer and poet Theodor Storm. Throughout his life Goethe was his ideal not only as a poet but also as “the wisest man of the century.” His perfect command of English, French, and Italian did much to make his journeys of research successful; he quoted Shakespeare in his letters almost as often as Goethe.

To many critics Mommsen's glorification of the dictator Caesar and his disparagement of Caesar's opponents, Pompey and Cicero, seem strangely inconsistent with his political liberalism. He tried to make his critics understand that he had praised Caesar only as a saviour of the decaying state; yet Mommsen's admiration for the autocrat reveals something of his own character. He himself was an autocrat in his own branch of scholarship, adopting a manner that his opponents labeled “caesarism.” At the same time, however, he had an unusual need for the fellowship of like-minded men. He held personal contacts to be one of the most important elements of life; indeed, it might be said that he had a genius for friendship. Yet it was mostly a friendship with men who looked up to him. With anyone who considered himself Mommsen's equal, a friendly relationship was not likely to last long.

It was only as a member of the Prussian Academy of Sciences that Mommsen could pursue his project of publishing his collection of Latin inscriptions, and for this reason in 1858 he was offered a post in Berlin. In 1861 he also became a professor in the philosophy faculty at the university; because of his philological and historical interests he chose that faculty rather than that of law. As a teacher of Roman history and epigraphy—especially in his seminars—he trained many students who were laterto make their mark in these fields. The main part of his scholarly work was taken up with the continuation of the Corpus Inscriptionum Latinarum (published 1863 and after). He also acted as adviser on many other great scholarly enterprises, such as the Monumenta Germaniae Historica, the exploration of the limes (Roman border fortifications in southwestern Germany), the numismatic work of the Prussian Academy, and the Thesaurus Linguae Latinae. Even in old age his mind was open to the new demands of scholarship, as shown by his interest in the new study of papyrology.

Mommsen's historical work was interrupted by his work on inscriptions; thus, the Romische Geschichte was never completed. Its first three volumes had been published in 1854–56. When, several decades later, in Berlin, Mommsen set out to complete his history, he abandoned the idea of writing the fourth volume, which was to contain the history of the emperors, because he felt that he would not be capable of writing it in the same brilliant style as his history of the republic. The fifth volume (1885) deals with the history of the Roman provinces in the first three centuries of the empire. No one but Mommsen could have depicted this period in so authoritative a manner, for no one else knew the nonliterary sources—the inscriptions and coins—as did he. The Romische Geschichte has been translated into English as The History of Rome, with the fifth volume entitled The Provinces of the Roman Empire.

The greatest monument to Mommsen's scholarship, the work which is of even greater significance for scholars than the Romische Geschichte, is Romisches Staatsrecht (“Roman Constitutional Law”), published in 3 volumes between 1871 and 1888. He himself said that if he were to be remembered by anything, it would be by this work. The Romans themselves never codified their constitutional law; Mommsen was the first to do it. His historical approach to classical scholarship led him to systematize the innumerable legal details upon which the Roman constitution was based and to explain this complex body of law through an understanding of its historical development. Only an individual who, like Mommsen, was grounded both in law and in the classics would be in a position to investigate the public law of the Romans, and only an individual trained to think in historical concepts could understand it.

In public law, criminal law stands side by side with constitutional law, and Mommsen's last great work, published in 1899, is Romisches Strafrecht (“Roman Criminal Law”).

When Mommsen, who had already become a mythical figure for his contemporaries, died just four weeks before his 86th birthday, he had attained what he had always wanted. The task which he had set himself to fulfill, according to his own almost superhuman standards, he had completed.