Thornleigh is a suburb of Sydney, Australia, within the Hornsby Shire local government area, about 22 kilometres north-west of the central business district. Its population in 2001 was 7,064.
Thornleigh is named after Constable John Thorn, who, along with Constable Horne, captured bushrangers Dalton and MacNamara on June 22, 1830, and were granted land as a reward; Horne's land became Hornsby, and Thorn's land Thornleigh.
Thornleigh is serviced by Thornleigh railway station (built in the 1880s), as well as Pennant Hills Road , one of Sydney's major thoroughfares. Several dozen shops and restaurants are located nearby. The rest of the suburb is partly residential, partly industrial; McDonald's has its Australian headquarters in Thornleigh.