LionClan was formed by Firestar and Tallstar joining their Clans, ThunderClan and WindClan, together to battle TigerClan (RiverClan and ShadowClan) for their right in the forest. Later, all four Clans joined together as LionClan to fight against BloodClan. LionClan was led by Firestar into battle, though none of the leaders took complete control of the combined Clan.
The modern LionClan was formed by ThunderClan and WindClan, after Tigerstar led TigerClan, made up of RiverClan and ShadowClan, to attack WindClan. ThunderClan came to help, but they were too late, and Tigerstar had already killed Gorsepaw, a WindClan apprentice. Firestar and Tallstar formed an alliance called LionClan, to face TigerClan.
However, Tigerstar was killed by the BloodClan leader, Scourge, who began an attempt to take over the forest with BloodClan. Firestar needed more warriors to fight BloodClan, so he went to RiverClan, where TigerClan was staying, and tried to convince them to join LionClan and fight off BloodClan. Leopardstar and Blackfoot decided to join RiverClan and ShadowClan to them. Blackfoot opposed at first, but after a talk with Leopardstar and the rest of the warriors, he agreed. Darkstripe, who died in the battle, had opposed the very idea of fighting BloodClan and left the camp after the decision was made, accusing them of being fools and predicts them being ripped apart when the battle comes. He fought on BloodClan's side, along with Jaggedtooth, who betrayed ShadowClan and went to BloodClan, as mentioned by Boulder.
The united four Clans trained, and went to face BloodClan. The leaders decided to have Firestar speak for them, and he led them into battle with BloodClan. Firestar killed Scourge, and the battle ended. After the battle was over, LionClan disbanded, and there were four Clans again.
Firestar knew he was to create LionClan because StarClan sent him a sign where the reflection of his head in a pool of water was turned into a lion's head. This prophecy is often referred to as the "Lion and Tiger Prophecy."
The Clans joined in the order of ThunderClan, WindClan, while RiverClan and ShadowClan joined at the same time.
They were heavily outnumbered by BloodClan in the fight at Fourtrees, but when Firestar killed Scourge, BloodClan admitted defeat and returned to the Twolegplace, giving victory to LionClan and ending the threat of BloodClan. They were chased out by WindClan back to the Twolegplace.
LionClan then split into four Clans again, led by their own leaders.