Magas was the son of Berenice and Philip, a Macedonian noble man, before Berenice remarried with the powerful Ptolemy I, founder of the Greek Ptolemaic dynasty in Egypt. Magas was a half-brother to their son, Ptolemy II Philadelphos, the second Ptolemy ruler of Egypt.

Magas received the governorship of Cyrene from his mother Berenice. Following the death of Ptolemy I however, Magas tried on several occasions to wrestle independence for Cyrene, until he crowned himself king around 276 BC.

Magas then married Apama, the daughter of the Seleucid king Antiochus I, and used his marital alliance to foment a pact to invade Egypt. He opened hostilities against his half brother Ptolemy II Philadelphos in 274 BC, attacking Egypt from the west, as Antiochus I was attacking Palestine. However Magas had to cancel his operations due to an internal revolt of the Libyan nomad Marmaridae. In the east, Antiochus I suffered defeat against the armies of Ptolemy Philadelphos.
Magas at least managed to maintain the independence of Cyrene until his death in 250 BC, upon which the kingdom was almost immediately reabsorbed by Ptolemaic Egypt.