The current parish dates from 1771, when the parishes of Houston and Killellan were united. The former parish church in the nearby hamlet of Killellan was known as the Church of St Fillan, which the name of the settlement derives. The former church building was left redundant by the unification and was abandoned. The ruin, dated 1635 but believed to have originated in the 10th or 11th century, still stands some west of the centre of Houston on the Barochan Cross Road. This church is now held by the Kilallan Kirk Preservation Trust following its donation by Elderslie Estates in 2005 and is a scheduled historic monument. An annual evening service is held amongst the ruins in July by the congregation of the united parish. The church's manse, which is immediately adjacent to it, also finds continued use as a private residence and is thought to be the oldest existing dwellinghouse in Renfrewshire. St Fillan's Holy Well and St Fillan's Seat are located near to the old kirk.