He was the eldest son of Ptolemy I Soter (ruler of Egypt) and his third wife Eurydice (daughter of Antipater). His younger brother Ptolemy II became heir apparent and, in 282 BC, the new pharaoh. Ptolemy Keraunos had left Egypt and arrived at the court of Lysimachus, the king of Thrace, Macedonia, and part of Asia Minor. His half-sister Arsinoe II was wife of Lysimachus.
After Lysimachus' defeat and death in the battle of Corupedium in 281 BC, against Seleucus I Nicator, Ptolemy Keraunos murdered Seleucus I. In 281 BC he made an alliance with Pyrrhus of Epirus. Ptolemy asked his half-sister Arsinoe II, the widow of Lysimachus, to marry him. After the ceremony he killed Arsinoe's two younger sons and Arsinoe II herself fled to Egypt and married her own brother Ptolemy II.
Ptolemy I (367 - 283 BC reigned 305 - 283 BC), founder of the dynasty of the same name, son of Lagus, a Macedonian nobleman of Eordaea, was one of Alexander the Great's most trusted generals, and among the seven "body-guards" attached to his person.
In 312 BC Ptolemy, with Seleucus, the fugitive satrap of Babylonia, invaded Palestine and beat Demetrius, the son of Antigonus, in the great battle of Gaza.