According to the 4th-century historian Rufinus (x. 9), who cites Frumentius' brother Edesius as his authority, as children (c.316) Frumentius and Edesius accompanied their uncle Metropius on a voyage to Ethiopia. When their ship stopped at one of the harbors of the Red Sea, people of the neighborhood massacred the whole crew, with the exception of the two boys, who were taken as slaves to the King of Axum. The two boys soon gained the favour of the king, who raised them to positions of trust and shortly before his death gave them their liberty. The widowed queen, however, prevailed upon them to remain at the court and assist her in the education of the young heir Erazanes and in the administration of the kingdom during the prince's minority. They remained and (especially Frumentius) used their influence to spread Christianity. First they encouraged the Christian merchants present in the country to practise their faith openly; later they also converted some of the natives.
When Erazanes came of age, Edesius returned to Tyre where he stayed and was ordained a priest. Frumentius, on the other hand, eager for the conversion of Ethiopia, accompanied Edesius as far as Alexandria, where he requested Athanasius, Patriarch of Alexandria, to send a bishop and some priests to Ethiopia. By Athanasius' own account (Athanasius, Epistola ad Constantinum), he believed Frumentius the most suitable person for the job and consecrated him as bishop either in 328 or, according to others, between 340-46. Frumentius returned to Ethiopia, erected his episcopal see at Axum, baptized King Azanas, who had meanwhile succeeded to the throne, built many churches, and spread Christianity throughout Ethiopia. The people called Frumentius Abuna (Our Father) or Abba Salama (Father of Peace), titles also traditionally given to the head of the Ethiopian Church.
A letter exists from the Emperor Constantius to King Azanas and his brother Saizanas, in which he vainly requested them to substitute the Arian bishop Theophilus for Frumentius.
The Roman Catholic church celebrates the feast of Frumentius on October 27, the Eastern Orthodox on November 30, and the Coptic on December 18. Ethiopian tradition credits him with the first Ethiopian translation of the New Testament.