It is named after St John's Cathedral[1] in Parramatta, New South Wales which opened in 1803, on which the civil parish is roughly centred.

The parish is shaped like a "T", with the northern part stretching from Toongabbie to the western shore of Homebush Bay.

Historically and until the reorganisation of local government areas in 2016, St John Parish largely corresponded with the boundaries of the City of Parramatta.

Since the 2016 reorganisation, the northern part of the parish (the horizontal stroke of the "T") is largely in the City of Parramatta, while the southern part is largely in Cumberland City Council.

It includes the suburbs of Merrylands, Parramatta, Silverwater, Westmead, Fairfield East, Guildford and Wentworth Point.