Agis marched the whole of the Spartan army, together with the neodamodeis and everyone who was able to fight in Sparta out to Tegea where he was joined by his allies from Arcadia, and he sent for help from his northern allies, Corinth, Boeotia, Phocis, and Locris. However, the northern army could not arrive quickly to the scene, as they had not expected the call and would have to pass through enemy territory (Argos and Orchomenus). On the whole, the army of the allies of Sparta would have numbered around 9,000 hoplites.
In the meantime, the Eleans decided to attack Lepreum, a contested border town with Sparta. So, they chose to withdraw their contingent of 3,000 hoplites. Agis took advantage of it and sent a sixth of his army, with the youngest and the oldest hoplites home to guard Sparta proper. They were called back soon after, as Agis or the xymbouloi realized that the Eleans would soon be back on the side of the Argives, but did not arrive in time for the battle.
Agis could have bided his time inside the walls of Tegea, waiting for his northern allies. However, he was already discredited and could not show the slightest sign of shying away from battle. So, he invaded and ravaged the territory around Mantinea, about ten miles north of Tegea and a member of the Argive alliance, in order to force a pitched battle with the Argives and their allies. The Argive Army, however, was situated on ground 'steep and hard to get at' and would not be drawn in battle, probably because the grain harvest had already been stored (the battle probably took place in the end of September 418). Agis, who was desperate for a victory to redeem his embarrassment at Argos, charged ahead; but according to Thucydides, when the armies had closed to a stone's throw, "one of the elder Spartans (the xymboulos Pharax, according to Diodorus) advised him not to try and correct one error (his former defeat) with another. The Spartans therefore retreated, and went off to find a way to draw out the Argive army to a battle. So, they diverted the Sarandapotamos River to the bed of the smaller Zanovistas river, or, they just filled up the sinkholes in which Zanovistas flowed, in order to flood the Mantinean territory.
Instead of allowing Mantinea to be flooded, the Argive army moved quicker then the Spartans anticipated, as the Argive hoplites were very angry at their generals for not pursuing the Spartan army and accused them of treason. They surprised their enemies by drawing up as the Spartans emerged from a nearby wood. The Spartans quickly organized themselves, with no time to wait for their other allies. Brasidas' veterans (Brasidas himself had been killed at the Battle of Amphipolis), and the Sciritae (an elite unit of Spartan troops) formed the left wing, the Spartans, Arcadians, Heraeans and Maenalians in the centre, and the Tegeans, who were fighting for their homeland took the position of honour on the right wing. The Argive lines were formed by the Mantineans on the right, the Argives in the centre, and the Athenians on the left. Thucydides did not know the exact numbers of men on each side, but estimated that there were about 9,000 men on the Spartan side (the Spartan army must have numbered about 3,500, with 600 Sciritae, about 2,000 neodamodeis and Brasideans and about 3,000 Arcadians on the whole) with somewhat fewer men on the Argive and Athenian side (about 8,000), according to Donald Kagan. Other scholars, such as V.D. Hanson, give slightly bigger numbers.
As the battle began, each side's right wing began to outflank the other's left, due to the erratic movements of each hoplite trying to cover himself with the shield of the man beside him. Agis tried to strengthen the line by ordering the Sciritae and his left to break off contact from the rest of the army and match the length of the Argive line. To cover the void created, he ordered the companies of Hipponoidas and Aristocles to leave their positions in the center and cover the line. This however was not achieved, for the two captains were unable, or unwilling to complete these manoeuvres on such short notice. This kind of manoeuvre was unprecedented in the history of Greek warfare. Donald Kagan considers it a most ill-advised move and gives credit to the two captains for disobeying orders that would have probably lost the battle for the Spartans. Others consider it a move that could have succeeded.
In any case, the Mantineans and the right part of the Argives, the elite Argive Thousand entered the gap and routed the Brasideans and the Sciritae and pursued them for a long distance. In the meantime, the Tegeans and the regular Spartan army routed the Athenians and the Arcadians that formed the left part of the Argive army. Most of them 'did not even stand to fight, but they fled as the Spartans approached; some were even trampled in their hurry to get away before the enemy reached them'.
Then, the Spartans turned left and broke the Argive right which fled in total disarray. The Spartans did not pursue the enemy for long after the battle was won.
The Spartans sent an embassy to Argos and the Argives accepted a truce by the terms of which they gave up Orchomenus, all their hostages and joined up with the Spartans in evicting the Athenians from Epidaurus. They also renounced their alliance with Elis and Athens. After deposing the democratic government of Sicyon, the Argive Thousand staged a coup against the democratic rule of Argos, where the democrats' morale was low, because of the bad performance of the common army and the Athenians in the battle.
In more general terms, the battle was a considerable boost to the Lacaedemonians morale and prestige. For, after the disaster at Pylos, they had been considered cowardly and incompetent in battle. Their success at Mantinea marked a reversal of the trend and a realization of the Greeks of the near-invincibility of the Spartans in hoplite combat.