He was born to an aristocratic Roman family living in Spain: his father Severian was Duke of Cartagena. Saint Leander embraced monastic life as a young man in Seville, capital of the Visigoths, who had embraced Arianism and caused the Arian heresy to dominate throughout Spain. Leander became a leading figure in the struggle to restore his land to Orthodoxy, founding a school in Seville to promote the Orthodox faith. In 583 he travelled to Constantinople to seek the Emperor’s support for the Spanish Orthodox; while there he met St Gregory the Great (the future Pope of Rome), with whom he formed a lifelong friendship. On his return to Spain, Leander was made Bishop of Seville.
One of the holy bishop’s converts was Hermengild, one of the sons of the Arian king Leovigild. When Hermengild rose up against his father in the name of Orthodoxy, Leovigild launched a violent persecution of the Orthodox throughout his kingdom. (Leovigild had his son imprisoned, then executed on Pascha Day of 586.) By God’s grace, at the very height of the persecution Leovigild fell mortally ill, repented, and embraced the true Faith; at his urging his son and successor Recared converted to Orthodoxy and convened the Third Council of Toledo in 589, at which he proclaimed that the Gothic and Suevic peoples were returning to the unity of the One Church. Saint Leander presided at the Council, and devoted the rest of his life to educating the (mostly) newly-Orthodox people of Spain in the Faith. It was he who established the early form of the Mozarabic Liturgy. He reposed in peace on March 13, 600. (He is venerated on this day because his name was incorrectly placed on February 27 in the Roman Martyrology.)