The parish church of St Sannan was first mentioned in the Norwich taxation of 1254 but was rebuilt in 1778 and restored in 1879.
The ancient parish of Llansannan comprised the townships of Treflach, Mostyn and Hendreunig, Llansannan, Rhydeidon, Beidiog, Penaled (or Pennant Aled), Llysaled, Archwedlog, Chwibren, Hescin, Deunant, Arllwyd, and Grugor. In 1855, the new parish of Bylchau was created; and the township of Grugor, with portions of Archwedlog and Deunant were transferred from Llansannan to Bylchau. Later part of a grouped parish along with Bylchau and Nantglyn, today Llansannan forms part of the Parish of Petryal along with Llanfair T.H., Llangernyw and Gwytherin.