He sided with Cicero during the Catilinarian conspiracy, did his utmost to prevent his banishment, and subsequently supported his claim for the restoration of his house.
On the next day Cicero attacked Catiline so vigorously in the senate (in his first Catilinarian oration) that he fled to his army in Etruria.
Little else is known of him except that he declared in favour of the death punishment for the Catilinarian conspirators.
On the outbreak of the Catilinarian conspiracy, Antonius was obliged to lead an army into Etruria, but handed over the command on the day of battle to Marcus Petreius, on the ground of ill-health.
For a time he co-operated with P. Clodius Pulcher, probably out of hostility to Cicero, who had caused Lentulus Sura to be put to death as a Catilinarian; the connexion was severed by a disagreement arising from his relations with Clodius's wife, Fulvia.