[3][4] Ffestiniog was a parish in Cantref Ardudwy; in 1284, Ardudwy became part of the county of Merionethshire, which became an administrative county in 1888.

[5] Mary Evans (1735–89) founded a sect in Ffestiniog around 1780, whose believers held that she had married Christ in a ceremony held in Ffestiniog church.

Ffestiniog became a community with an elected town council.

The form -iog with an additional /i/ can be explained by the preceding element, which is a personal name ending with -i-us : Festinius.

A Roman Inscription in Chester honours Lucius Festinius Probus;[8] Chester was the main Roman base for North Wales, so this is probably he.