Mount Colah is a suburb of northern Sydney, in the state of New South Wales, Australia 29 kilometres north of the Sydney central business district, in the local government area of Hornsby Shire.
Mount Colah is 5 km north of Hornsby, the nearest major town centre. It is one of the most northerly suburbs of Sydney and is where the "Welcome to Sydney" sign is located. Mount Colah is the second highest suburb in Sydney by elevation. Considered one of Sydney's leafier suburbs, streets are clustered around the Pacific Highway. Mount Colah is bordered to the east by the Ku-ring-gai Chase National Park.
Mount Colah varies in altitude from 68 m to about 206-217m above sea level.