Frías was Minister of Foreign Relations of President José Ballivián (1841-1847) and a steadfast supporter of civilian rule and the primacy of laws. He was named President by Congress upon the death of dictator Agustín Morales in November 1872. His task was to call free elections as soon as possible. He did so, and in May 1873 transferred power to the winning candidate, Adolfo Ballivián, the son of the former President and war hero, José Ballivián. Unfortunately, Adolfo Ballivián soon fell ill with cancer and died in February 1874, after only nine months in office. At that point, Tomás Frías became President again by virtue of his being head of the Council of State, in accordance to the Constitution then in effect. As Ballivián's legal successor, his term in office was projected to run until 1877.