<lann> 'enclosure (religious)' + <Hernow> personal name
Historic forms:
Lanherueu 1086, Lanhern c1187, Sanctus Malganus 1206, 1260, Sanctus Mauchanus of Lanherno 1257, Ecclesia de la Herne 1279, Seyntmogan 1284, Lanhorn 1284, Sanctus Mauganus, Sanctus Mauganus de Laherne 1309, 1327, Lanhern 1349, 1380, Sanctus Mauganus c1410, Mawgan 1543, 1656.