Thornleigh is a suburb located on the Upper North Shore of Sydney, in the state of New South Wales, Australia, 25 kilometres north-west of the Sydney central business district in the local government area of Hornsby Shire.. Thornleigh is bounded to the north by Waitara Creek. Thornleigh borders the suburbs of Normanhurst, Westleigh, Hornsby, Wahroonga and Pennant Hills. It is considered part of the Northern Suburbs of Sydney.
Thornleigh offers great district views and the topography varies greatly with many properties built around bushland settings and into the hills to afford the great views. Top streets are considered the top of Tillock Street and Wanawong Drive for some of the suburbs best views and proximity to the train station.