Mount Colah is an outer suburb of Northern Sydney, in the state of New South Wales, Australia 24 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.