As a tribune of Concilium Plebis in 137 BC he put forward and passed the Lex Cassia Tabellaria (a measure to change the voting system to one of secret ballot). In 127 BC he was consul with Lucius Cornelius Cinna and in 125 BC he was elected censor. He was renowned for severity as a iudex and gained fame by formulating the question "Cui bono?" ("Good for whom?", or "Who benefits?") as a principle of criminal investigation.
In 114 BC he was appointed special inquisitor to the case of three Vestal Virgins accused of unchastity. He condemned and put to death two of them, who were acquitted by Pontifex Maximus L. Caecilius Metellus Dalmaticus, as well as the men involved.