Alcathous was the son of Pelops and Hippodamia. He first married Pyrgo and afterwards Euaechme, and was the father of Ischepolis, Callipolis, Iphinoe, Periboea,and Automedusa.
after Euippes, son of king Megareus, was killed by the Cythaeronian lion, Megareus, whose elder son Timalcus had likewise fallen by the hands of Theseus, offered his daughter Euaechme and his kingdom to anyone who could slay the lion. Alcathous undertook the task, killed the lion, and thus obtained Euaechme for his wife, and afterwards became the successor of Megareus. In gratitude for this success, he built a temple of Artemis and Apollo in Megara. He also restored the walls of Megara, which had been destroyed by the Cretans.
Ischepolis was killed during the Calydonian hunt in Aetolia, and when his brother Callipolis hastened to carry the sad tidings to his father, he found him engaged in offering a sacrifice to Apollo. Thinking it unfit to offer sacrifices at such a moment, he snatched away the wood from the altar. Alcathous imagining this to be an act of deliberate sacrilege, and killed his son on the spot with a piece of wood. The acropolis of Megara was called by a name derived from that of Alcathous.