Arsinoe was born to Pharaoh Ptolemy I Soter(the founder of the Ptolemaic Dynasty) and his second queen, Berenice I. Arsinoe had an older brother, Ptolemy Philadelphus and a sister, Philotera. She had three older half-siblings on her mother's side: Magas, king of Cyrene, Antigone, Queen of Epirus and Theoxena, Queen of Syracause. Through her father's first marriage wirh Eurydike she had five siblings: Ptolemy Keraunos, Lysandra, Ptolemais, Meleager, and Argeaus. There was a battle struggle between Queen Berenice and Queen Eurydike whose son would succeed Soter as King.
When she was fifteen, Arsinoe married Lysimachus, King of Thrace. Her brother Philadelphus married Lysimachus' daughter also named Arsinoe. Arsinoe had three sons with Lysimachus: Ptolemy I Epigone, Lysimachus, and Philip. But before any of them could take the throne, she had to expel her husband's older children. His son Agathocles married her half-sister Lysandra and had issue. His daughter Eurydike was put to death after she sided with her husband Antipater.
In 282 BC, Ptolemy I Soter died and Ptolemy II Philadelphus ascended to the throne as King. Ptolemy Keraunos came to live in Thrace with his sister and her husband. Arsinoe's accused Agathocles of trying to steal the throne. Lysimachus executed Agathocles. Thus clearing the way for Arsinoe's own son. But Lysandra and Keraunos allied with Seleucus I Nicator and made war on Lysimachus and Arsinoe.
During the Battle of Corepedium, Lysimachus was killed. Thus Thrace became part of the Seleucid Empire. However Keraunos betrayed Seleucus and killed him, than sided with Arsinoe against Lysandra. Arsinoe married her half-brother, citing ancient egyptian traditions, ruling over Thrace, Macedonia and parts of Asia. However soon the siblings were at each other's throats. Arsinoe tried to drum up support so that her sons could rule but Keraunos found out and killed Lysimachus II and Philip. Arsinoe and her oldest son escaped to Alexandria, Egypt where her older brother ruled as Pharaoh.
Though having lost Thrace, Arsinoe tried her best to incorporate herself into her brother's inner circle. When Arisnoe I, Ptolemy II's wife tried a coup against him and failed, he divorced her and sent her away to Thebes. Arsinoe married her brother and was crowned co-regent, becoming the first female Pharaoh of the Ptolemaic Dynasty.