St Teilo's church is believed to have been built during the late 12th or 13th century on the site of an earlier Celtic church. Over the ensuing centuries, the building was altered and extended.
The oldest parts of the present structure are the nave and chancel. During the 14th century, small chapels were built onto the north and south sides of the chancel, and during the late 14th or early 15th century, the church's capacity was increased by the addition of an aisle to the south side of the nave. The old south wall was replaced by two arches, with a third arch opening into the chancel, and finally, a porch was added to the entrance door leading into the south aisle of the church.