St. Tysilio was a bishop, prince and scholar, son of the reigning King of Powys, Brochwel Ysgithrog, maternal nephew of the great Abbot Dunod of Bangor Iscoed and an ecclesiastic who took a prominent part in the affairs of Wales during the distressful period at the opening of the 7th century.

Tysilio probably started his career in Trallwng Llewelyn (Welshpool) and afterwards took up residence in Meifod where he was associated with Gwyddvarch and St Beuno. He founded the second church in Meifod - the Eglwys Tysilio. His feast day, or gwyl-mabsant, was the 8th November which was also the date of the patronal festival and "wakes" in the nearby parish of Guilsfield, where a holy well was dedicated to him - the Fons Tysilio.

In the 7th century, the Church of Wales was still separate from the control of the Church of Rome, so Tysilio was canonised by the authority of the Bishop of the diocese of St. Asaph.

Today the name of Tysilio is remembered in several church and place names in North Wales; most famously in the longest place name in the United Kingdom, Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch, which translates into English as "Saint Mary's Church in the hollow of the white hazel near a rapid whirlpool and the Church of St. Tysilio of the red cave".