Phrontis and his brothers were raised in Colchis, but after their father died, he and his brothers set out to avenge their father’s ill treatment in the hands of king Athamas of Orchomenus and were stranded on the Island of Ares the War God in the Black Sea until they were rescued from the island by Jason and the Argonauts. Once Jason received discovered that Phrontis and his brothers were grandsons of King Aeëtes of Colchis, Jason convinced Phrontis and his brothers to return with him to Colchis and help him to obtain the Golden Fleece. Jason also questioned Phrontis and his brothers on the layout and security of the land. After the Fleece was retrieved from Colchis, Phrontis and his brothers returned with the Argo's crew to Greece. His brothers were Cytissorus, Argus and Melas, and, according to some accounts, Presbon was another one of his brothers.
Another story of Phrontis transpired many years after the Argonautical travels and involves Phrontis' brother Melas as a traveling merchant who was apprehended and kept in captivity by the oppressive king Talycrates (Ταλυκρατης) of the city of Ionetrea (Ιονητρεια) in the Caucasus along the Cyrus River. Many other traveling merchants who journeyed through the Caucasus were held in captivity as well. Talycrates hoped that, by holding captive as many merchants from Greece and Anatolia as possible, he could economically weaken the wealthy city-states in those regions, giving him a great advantage of conquest. Once Phrontis received word of his brother's captivity, he felt compelled to travel the long journey to rescue his brother. Phrontis prayed to Zeus for help and Zeus gave Phrontis' horse Tempestris the wings of an eagle (Zeus' sacred animal), enabling the horse to fly over the high Caucasus Mountains and carry Phrontis safely to Ionetrea. After arriving at Ionetrea, Phrontis freed his brother Melas, and many other Greek and Anatolian traveling merchants, from prison bonds. Afterward he helped the inhabitants of Ionetrea to revolt and to overthrow and kill Talycrates, thus liberating the Ionetreans from his tyrannical oppression. An Ionetrean maiden named Lystra joined Phrontis and Melas on the journey back to Greece, and Phrontis married Lystra in Greece.