According to the most well-known version, known from at least the 16th century (recounted in André de Resende's Breviário Eborense, 1548[4]), he was born in Rome and, converting to Christianity, went to Jerusalem, where he was able to meet Jesus Christ and be one of His disciples.

He preached in the Alentejo region, where he converted many people, and arrived at Évora, where he established a Christian community, presided over by him as the first bishop.

The Roman persecution of Christian communities would have Mantius imprisoned; after refusing to offer sacrifices to the pagan gods, he was martyred near Évora,[3] traditionally in the site nowadays occupied by the civil parish of São Manços.

In 1070, Gutierre Téllez de Meneses would have had a dream in which Saint Mantius told him to take his relics to Castile.

Thus the relics arrived at what today is Villanueva de San Mancio, in the province of Valladolid, near Medina de Rioseco, Spain; the authorities would not let him enter the town with the saint's remains, so the knight founded a monastery on the outskirts, with the support of the Benedictine monks of Sahagún, where the relics were left — the town subsequently grew around the monastery.