Antonius was a Capitol Minister, and a member of the Presidential Cabinet, working closely alongside President Snow. Although his role within the Presidential Cabinet is not clear, his appearances have shown that he is an entrusted and high ranking member within the Capitol's political system.
In Mockingjay Part 1, he is seen informing President Snow of Katniss Everdeen's presence in District 8, as well as advising him that hovercraft could be deployed from the border of District 11. He is not mentioned within the book series. He is later seen alongside Snow and Egeria when the rebels destroy the hydroelectric dam in District 5 which causes the power in the Capitol to be cut.
In Mockingjay Part 2, Antonius is shown during a meeting with Snow and other members of the cabinet, debating future plans for the battle ahead. After contesting Snow's view, he begins choking to death after drinking or eating, hinting that Snow may have used poison on him.
Antonius possesses a lethal personality such as that of President Snow. However, as depicted in Mockingjay--Part 1 & 2, he lacks intellect. In Mockingjay--Part 1, Antonius informs Snow that Katniss and the then unnamed Squad 451 have visited a temporary hospital in District 8, and suggests that the Capitol launch hovercraft to ambush the Squad in its retreat back to District 13. However, Snow advises Antonius to "kill the wounded", meaning everyone within the hospital Katniss had visited. Though this order was carried out, it was not without repercussions as Katniss, Gale, and Commander Paylor destroyed the sent Capitol hovercraft and the brutal spectacle itself, aired live at Snow's request, increased momentum within the rebels of the districts and intensified the rebellion in part thanks to Katniss's heartfelt speech propo.
In Mockingjay--Part 2, Antonius was likely responsible for ordering numerous attacks on rebels of District 2 who attempted to conquer the Capitol's arsenal within the mountains. When the rebels launched a more uniquely devised attack, the Capitol's arsenal was suppressed and caved in with many Peacekeepers still trapped inside. Most of these Peacekeepers were sent at Antonius's request, many of whom were, as Snow called them, "best" of the Capitol. As a result of his arrogance in placing a large amount of Peacekeepers within the arsenal, Antonius was poisoned to death, with Snow accusing him of having a habit of "burying people before they are dead".
Antonius's stunt with having an extreme amount of Peacekeepers protecting the arsenal cost the Capitol the victory of the rebellion Snow was hoping for as he (Antonius) had inadvertently weakened the Capitol and allowed the rebels to gain more ground and the upper hand.