Northumberland County is one of the original Nineteen Counties in New South Wales, now one of the 141 cadastral divisions of New South Wales. It included the area to the north of Broken Bay, which compasses the Central Coast region and Lake Macquarie as well as Newcastle (Greater Newcastle) in the Hunter region. It was bounded by the part of the Hawkesbury River to the south, the Macdonald River to the south-west, and the Hunter River to the north. Note that Northumberland County should not be confused with the former Northumberland County Council, a county council which existed between 21 July 1948 and 19 December 1963, and which despite the shared name was a legally distinct entity, with a distinct (albeit partially overlapping) territory, the Northumberland county district.
Northumberland County was named after the English Northumberland, and named by Lieutenant Charles Menzies, commandant at Newcastle, about 1804.
In 1852 it had an area of and population of 15,207, and was described as being long and wide and the main coal region of the colony.
Northumberland County contains the following parishes: