Thornleigh is a suburb in the Upper North Shore region of Sydney in the state of New South Wales, Australia.

Thornleigh is located 22 kilometres north-west of the Sydney central business district in the local government area of Hornsby Shire.

[2] The area bounded near the train station and the Comenarra Parkway include early examples of Federation and Californian Bungalow style properties.

Settlers moved into the area in the 1830s and among them were James Milson, Patrick Duffy, John Thorn and Samuel Horne.

Land sales posters used this as an attraction for prospective settlers describing the area as "beautifully situated (and) surrounded by magnificent orchards, the fruit from which affords a splendid proof of the fertility of the soil and mildness of the climate".

Eventually part of this land was purchased by the wealthy Friend family who built the Windyhaugh property on Duffy Avenue, which was later used as the first Presbyterian Fellowship Union camp in the Commonwealth.

[6] Thornleigh railway station opened on 17 September 1886 where the local produce (mainly citrus fruits) was exported to the city markets.

[7] The Thornleigh School of Arts opened in 1890 and was demolished in 1980 along with many other locations of historical significance in the suburb throughout the commercial development of the area in the 1970s and 1980s.

During the Great Depression of the 1930s, unemployment was a problem in the area, so a local woman named Lorna Brandt raised money for the construction of a walking track near the Lane Cove River as a way of providing relief work.

Thornleigh Marketplace containing a supermarket and speciality stores was constructed in 2005 and includes: A post office, butchers, hair and nail salons, pharmacy, newsagency, gym, sushi bar, cafes, optometrist and Woolworths.

American multinational fast food restaurant chain McDonald's have their Australian headquarters and flagship store in Thornleigh.

[17] The soccer club known as Thunder FC consists of teams ranging from Under 6s through to All Age divisions playing in the Gladesville Hornsby Football Association.