Hornsby, in the parish of South Colah, is about 180 metres above sea level and is the administrative centre of Hornsby Shire.
The first inhabitants of the area were from the Dharug language group.
Timber-getting was the first industry around Hornsby, felling the blue gums and ironbarks that grew along the ridges. One of the first permanent residents, Thomas Edward Higgins, settled in the district in 1832 although he had been granted land in Old Mans Valley in 1824. He started selling timber from the valley in 1835 and also made a living from farming the rich alluvial soils and establishing orchards on the slopes of Old Mans Valley, so named because of the 'old man' kangaroos that grazed there. Now called Hornsby Valley, it is the site of an ancient volcano and was quarried for more than 100 years, producing gravel and road base. Geologically it is of interest because of its unusual volcanic diatremes or plugs.
The area has had various names. Jack's Island – as it was known until it officially became Hornsby at the turn of the century – was an 'island' of settlement surrounded by bush. Lorna Ollif suggested that the name came from the number of settlers whose given name was Jack.  The land closer to Waitara was called Sandy Bar. In 1838, there were only four houses in the parish of South Colah. Mr Codie was one of the householders who lived at Jack's Island. Other people settled nearby and the township grew.
The capture of two bushrangers on the Windsor Road in 1830 resulted in the naming of the suburbs of Hornsby and Thornleigh. Police Constables John Thorn and Samuel Horne were rewarded with grants of land for shooting the bushranger John McNamara and capturing his accomplice. Constable Thorn's land later became known as Thornleigh. Constable Horne's land – 320 acres (130 hectares) 2.5 kilometres from present-day Hornsby – extended from Thorn's grant at Thornleigh along Pennant Hills Road to Pearces Corner.
A village soon developed on the land, and came to be known as Hornsby Place. Although Horne never lived in the area, the name remained until the early 1890s.
The arrival of the railway
On 17 September 1886, Hornsby railway station was opened, but it was in Jack's Island, three kilometres to the north of the old Hornsby village. Landowners there had powerful connections in government, and agitated for the railway line under construction through the northern suburbs to be extended to Hornsby. In 1890, the north shore railway line from St Leonards terminated at Hornsby station, forming a junction for the north shore and main line railways. On 23 October 1894, the station was renamed Hornsby Junction.
The residents of the old Hornsby village then petitioned for a station of their own. In 1895 a platform was built at Hornsby village and called Hornsby. People often confused this with Hornsby Junction and got off at the wrong stop, so Hornsby village was informed its platform had to be renamed. After some debate, Hornsby village and its station were renamed Normanhurst, and the area around Jack's Island was called Hornsby. Hornsby Junction station reverted to its former name of Hornsby.
Hornsby developed as a railway town, providing work for fettlers and other railway employees, shopkeepers and publicans.
Land values soared as it became a popular residential area for families of businessmen who commuted to work in the city. The area was advertised as comparable to the lower Blue Mountains with a healthy climate away from the smog of the city.
The first school in Hornsby was opened in 1883 on land immediately behind Hookham's Corner. It consisted of a tent with a board floor.
An early industry was the blue metal and gravel quarry, in operation from about 1906, which extracted road base from Old Mans Valley. Others included a sawmill in Jersey Street, started by Councillor PA James in the early 1900s; Fowler's Pottery, erected just before the start of World War I at Hookhams Corner; and Bullock's pipe works at Asquith.
A telephone exchange was opened in 1897, electricity was provided to Hornsby in 1923–24 and the railway was electrified in 1928. The population continued to grow rapidly, and in 1930 Hornsby Girls High School opened, followed by Hornsby Hospital in 1933. 
In 1970, workmen excavating for an extension to the council chambers and the construction of a library unearthed a cannon, built in 1877. It was later put on display at the Victoria Barracks in Paddington.
Since the end of World War II, there has been steady growth in population. In 1961, Westfield opened its first Shoppingtown at Hornsby and the shopping centre is now one of Sydney's largest. The hub of the centre is a water sculpture by Victor Cusack, which was unveiled in 1993.
The 1891 census counted 423 residents in the Hornsby area. By 1901, the population had increased to 1818 and in 2001 it had reached 18,504.