Cleomenes was born in Sparta to the future Agiad king Leonidas II and his wife Cratesicleia.

[1] Around 242 BC, Leonidas was exiled from Sparta and forced to seek refuge in the temple of Athena after opposing the reforms of the Eurypontid King, Agis IV.

Meanwhile, having started his reforms Agis went on a campaign near the Isthmus of Corinth, which presented Leonidas with an opportunity to regain his throne.

[6] In 229 BC, the cities of Tegea, Mantinea, Caphyae and Orchomenus, who were allied with the Aetolian League, joined Sparta.

[5][7][8] Later that year, the ephors sent Cleomenes to seize Athenaeum, a fort on the Spartan border with Megalopolis which was being disputed by both cities.

[7] In retaliation for fortifying Athenaeum, Aratus carried out a night attack on Tegea and Orchomenus but when his supporters inside those cities failed to help, he retreated, hoping to remain undetected.

[8] This success greatly encouraged Cleomenes, and when he heard that Aratus was attacking Sparta's ally, Elis, he set off to confront them.

Meanwhile, the Eurypontid King of Sparta, Eudamidas III, who was the son of Agis IV and Agiatis, died.

[12][13] Having bribed the ephors to allow him to continue campaigning, Cleomenes advanced into the territory of Megalopolis and started to besiege the village of Leuctra.

After gaining the support of his stepfather, he embarked with him on a whirlwind military expedition against his opponents, and when they requested to stay in Arcadia due to exhaustion he returned to Sparta to carry out his reform plans.

After having removed the ephors, who obstructed his political will, Cleomenes used the character of Lycurgus the lawgiver, which allowed him to legitimize the violence, and he began his reforms.

[17] For the first time the amount of produce the Helots had to surrender to each klaros-holder[clarification needed] was specified in absolute quantities rather than as a proportion of the annual yield.

Cleomenes completed his reforms by placing his brother, Eucleidas, in charge, making him the first Agiad king on the Eurypontid throne.

From Tegea, the Spartans advanced into Achaea, where Cleomenes hoped to force the League to face him in a pitched battle.

The Cleomenian reforms, although they were not intended to be applied to the defeated populations, was a significant factor behind the campaign successes of the Spartan king.

After this diplomatic failure, Aratus chose to negotiate with Antigonus III Doson, King of Macedonia, who decided to go to war against Sparta.

Despite numerous attempts to break through the defensive line and reach Lechaeum in the Corinth Isthmus, Antigonus' forces failed and suffered considerable losses.

[13] In 222 BC Cleomenes was defeated in the Battle of Sellasia by the Achaeans, who received military aid from Antigonus III Doson of Macedon.

Cleomenes returned to Sparta, advised the citizens to submit to Antigonus, and fled to Alexandria to his ally Ptolemy Euergetes of Egypt, hoping for assistance in regaining his throne.

The last to kill himself was Panteus, Cleomenes' favourite and right-hand man, who was ordered to make sure everyone was dead before taking his own life.