Old Toongabbie is located 29 kilometres west of the Sydney central business district, is only about 3.5 kilometres northwest of Parramatta CBD and in the local government area of the City of Parramatta.
Old Toongabbie is noted for being the third settlement set up after the British occupation of Australia began in 1788. It was founded as a Government Farm to grow food for the colony in April 1792. After eleven years, the government farm was closed and the land was given as grants to settlers and convicts who had done their time.
In 1860, the railway was extended to Blacktown but it took 20 years before any arrangements were made for trains to stop at Toongabbie. The first school in Toongabbie, opened on the 3rd of May 1886. By April 1911, the school closed due to low enrolments. The school reopened February the next year and has stayed open ever since.
The first post office in the area was opened after many years of campaigning by local residents in 1887 in a private house on Old Windsor Road and this arrangement continued until the 1960s. The first post master was a Mr Birks wand he was paid 25 pounds a year to manage the office and bring the mail bag over from Seven Hills on horseback each day. By 1922 the number of residents and businesses had grown sufficiently to support a second office in a weatherboard cottage in Wentworth Avenue, known as Toongabbie West. A purpose built office was opened in the main shopping area in Portico Parade in 1960 becoming Toongabbie Post Office whilst the old Toongabbie Post Office was renamed Old Toongabbie.
The suburb straddles busy Old Windsor Road, effectively creating two different areas. Both are dominated by single dwellings from the 1950s and 60s, though there is a small new development in hte suburb's east that is still being established.
From the 1990s part of this area was unofficially known as the locality of Constitution Hill. The suburb of Constitution Hill was officially recognised in 2007.