Mammy Johnsons River, a mostly perennial river of the Mid-Coast Council system, is located in the Mid North Coast and Upper Hunter regions of New South Wales, Australia.

Mammy Johnsons River rises on the southwestern slopes of the Great Dividing Range, below Winns Mountain, north northwest of Bulahdelah, and flows generally north, west and south, joined by four tributaries including Wards River, before reaching its confluence with the Karuah River north northwest of Stroud. The river descends 371 metres (1,217 ft) over its 55 kilometres (34 mi) course.

• Rivers of New South Wales
• List of rivers of New South Wales (L–Z)
• List of rivers of Australia

• "Karuah River and Great Lakes catchments" (map). Office of Environment and Heritage. Government of New South Wales.

32°15′S 152°03′E / 32.250°S 152.050°E / -32.250; 152.050