In 92 BC he defended his uncle Publius Rutilius Rufus, who had been unjustly accused of extortion in Asia.
Shortly afterwards he was prosecuted under the lex Varia, the law proposed by Quintus Varius Severus which was directed against all who had in any way supported the Italians against Rome, and, in order to avoid condemnation, went into voluntary exile.
Cotta obtained the province of Gaul, and was granted a triumph for some victory of which we possess no details; but on the very day before its celebration an old wound broke out, and he died suddenly.
Physically incapable of rising to passionate heights of oratory, Cotta's successes were chiefly due to his searching investigation of facts; he kept strictly to the essentials of the case and avoided all irrelevant digressions.
The fragments of Sallust contain the substance of a speech delivered by Cotta in order to calm the popular anger at a deficient corn supply.