According to 1 Maccabees he was a descendant of the Biblical Aaron, brother of Moses, but not in the high-priestly line; and being ambitious for the office of high priest, he traveled to Antioch to secure the assistance of the Seleucid king Demetrius I Soter, who had just overthrown Antiochus Eupator. Alcimus was of the Hellenizing party, and therefore bitterly opposed by the Maccabees.
Demetrius sent an army under Bacchides to establish Alcimus in the high priesthood at Jerusalem. The favor with which Alcimus was received by the Jews at Jerusalem on account of his Aaronic descent was soon turned to hate by his cruelties. When Bacchides and his army returned to Antioch, the Maccabean Simon attacked and overcame Alcimus, and drove him also to Syria. There he secured from Demetrius another army, led by Nicanor, who, failing to overcome Simon by treachery, attacked him directly, but was defeated and killed. A third and greater army, under Bacchides again, was dispatched to reinstall Alcimus. Simon was defeated and killed, Alcimus established as high priest and a strong garrison left in Jerusalem to maintain him. But he did not long enjoy his triumph, since he died soon after, while he was pulling down the wall of the temple that divided the court of the Gentiles from that of the Israelites.