Medius belonged to a noble Thessalian family, possibly related to the Aleuadae clan. According to Diodorus Siculus ("Library of History", XIV.82), his namesake grandfather was a dynast in Larissa in 395. He is first mentioned as commanding a trireme during the descent of the Indus River (Arrian, "Indica", 18) in 326 BC, but according to the historian Richard Billows it is likely that he was a member of Alexander the Great's expedition into Asia from the beginning, possibly within the ranks of the Thessalian cavalry. He enjoyed a high place in the personal favor of Alexander, becoming one of his "hetairoi" and most prominent courtiers during the Macedonian monarch's last days: he hosted the banquet where Alexander supped just before his final illness. Plutarch ("Moralia", 65) accuses him of being among the shameless flatterers who drove Alexander to some of his most reprehensible actions. In later literature, e.g. the "Alexander romance", he was considered a member of Antipater's conspiracy to poison Alexander, which took place during the banquet hosted at his house.