Aratus was born in 271 BC in the northern Peloponnese city-state of Sicyon. His father, Cleinias of Sicyon, had become head magistrate of the city jointly with Timocleidas after the murder of the previous tyrant. Shortly after government stabilised, Timocleidas died. In 264 BC, Abantidas murdered Cleinias, and proclaimed himself tyrant. The seven-year-old Aratus escaped the coup with the help of his aunt who, in the confusion, was able to hide the boy until nightfall and then smuggle him out of Sicyon to Argos. He was raised in Argos by friends of his father, living there until 251 BC.