The son of a village priest in Novgorod Province, Modestov trained initially at the Petersburg Pedagogical Institute to become a teacher. Instead, he became one of the leading figures in the history of Rome, beginning with the Neolithic Age in Italy. After retiring from New Russia University, he moved to Rome. Perhaps surprisingly for a leading classicist scholar, he opposed the reforms of Minister of Education Dmitrii Tolstoi, who in the 1870s returned the university curriculum to emphasize the ancient languages over history or philosophy. He also translated Tacitus into Russian.