Glen Forrest is a suburb within the Shire of Mundaring, south of John Forrest National Park, west of Mahogany Creek, east of Darlington, and north of the Helena River. Its northern boundary is determined by the Great Eastern Highway. Originally known as Smiths Mill after a prominent founder citizen, it is currently named after the first Premier of Western Australia, Sir John Forrest. The suburb is bisected by a disused railway track - the original route of the Eastern Railway - which is now known as the Railway Reserve Heritage Trail, and Nyaania Creek.