first bishop of London and the third archbishop of Canterbury (619–624), known for his missionary work and his diplomatic efforts between the Roman church and the churches of Britain.

Mellitus, a Roman and the son of a noble family, may have been the abbot of St. Andrew's monastery in Rome before he was summoned to the missionary field by Pope St. Gregory I the (Gregory I, Saint) Great. In 601 he was sent to England by Gregory to assist Augustine, the archbishop of Canterbury and the head of Gregory's first mission to the Saxons. The pope also sent a letter instructing Mellitus to tell Augustine and his fellow missionaries to destroy the Saxons' idols but to convert their pagan places of worship into churches (rather than destroy them) and to preserve local pagan festivals by turning them into Christian observances. These measures helped to make Christianity more acceptable and, consequently, greatly furthered the cause of the church in England.

Consecrated as a bishop (c. 604), Mellitus was directed to preach in the kingdom of the East Saxons. He baptized their king, Saberht, but failed to convert Saberht's sons. After Saberht's death, Mellitus was eventually banished by his sons, supposedly when he refused to give them the bread of Communion because they were unbelievers. After a brief exile in Gaul, he was recalled by Laurentius (Lawrence), archbishop of Canterbury, whom he succeeded in that office in 619. According to legend, Mellitus saved Canterbury from destruction by fire, the strength of his prayers summoning a great wind that drove the flames away from the city.