The former parish of Llandecwyn stretched from the estuary of the Afon Dwyryd at Pont Briwet to the hills of the Rhinogs.

It also included land across the River Dwyryd: the area of Cefn Coch and around Rhiw Goch and the road to Llanfrothen.

Most of the former parish is now part of the Bro Ardudwy ministry area,[1] which includes Harlech, a few kilometres to the southwest, and Barmouth.

Also within Llandecwyn, there is a Welsh Presbyterian chapel called Bryn Tecwyn which had weekly services until December 2017.

[11] The local landowners were the Wynn family of Maes y Neuadd, who were descended from the 13th Century Osbwrn Wyddel ('Osborn the Irishman').

[12] Other notable residents of Llandecwyn have included: Mary Evans (1735 – 1789), was a mystic and cult leader, known as 'y Fantell Wen' (Whitemantle).

She was either a servant at Maentwrog rectory, or lived at Breichiau between Llys Tecwyn Uchaf and Ceunant Llennyrch.

She claimed to be betrothed to Christ, and led a group of followers in ceremonies on Manod Mawr and other hills.