The St. Tysilio church is situated on the aptly named Church Island in the Menai Strait adjacent to the Menai Strait bridge and is reached by crossing a small causeway. The Welsh name for the island is Ynys Tysilio, meaning the Island of Tysilio.
It is thought that Tysilio was the son of the famous sixth century ruler of Powys, Brochfael Ysgythrog and rather than become a warrior as his father wished he turned to religion and built the St. Tysilio church in the 6th century.
It is widely accepted that the current church has replaced an earlier church or chapel on the same site during the 15th century.