Trelystan is a remote parish and township on the border of the historic county of Montgomeryshire with Shropshire. Trelystan now forms part of the community of Forden, Leighton and Trelystan in Powys. St Mary’s Church, Trelystan is sited 900 feet up at the S end of the Long Mountain to the east of Welshpool. Trelystan was a chapel of ease within the parish of Worthen and it also served the township of Leighton.