He was the second son of Attalus I Soter and queen Apollonis of Cyzicus,[1] and ascended the throne first as co-ruler alongside his ailing brother Eumenes II in 160 BC, whose widow Stratonice of Pergamon he married in 158 BC upon Eumenes' death.

Around 189 BC he led his forces to fight alongside the Roman Army under Gnaeus Manlius Vulso in Galatia.

[7] When his brother died in 159 BC, his nephew was too young to rule at the time, so he ascended the throne as regent and married Stratonice once again.

In the summer of 152, he, Ptolemy VI, Ariarathes V, and Rome, helped the pretender Alexander Balas to seize the Seleucid throne from Demetrius I[8] and in 149 BC, he helped Nicomedes II Epiphanes to seize the Bithynian throne from his father Prusias II.

[9] Attalus expanded his kingdom with the help of his good friend Ariarathes V of Cappadocia, and founded the cities of Philadelphia and Attalia (Antalya).