Tyslio was the second son of the reigning King of Powys, Brochwel Ysgithrog,[4] and the maternal nephew of the great Abbot Dunod of Bangor Iscoed.

Tysilio probably started his career in Trallwng Llywelyn (Welshpool) and afterwards took up residence in Meifod where he studied under Gwyddfarch.

His feast day, or gwyl-mabsant, was 8 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.

[8] Today Tysilio's name is remembered in several church and place names in Wales, including Llandysilio in Powys,[4] Llandissilio in Pembrokeshire, Llandysiliogogo in Ceredigion and Llantysilio in Denbighshire with Llantysilio Hall, a Grade II listed building.

It appears in the longest place name in the United Kingdom, Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch, part of which (shown bold here) means "the Church of St. Tysilio".