A. mearnsii is native to south-eastern Australia and Tasmania, but has been introduced to North America, South America, Asia, Europe, Pacific and Indian Ocean islands, Africa, and New Zealand. It has been introduced to numerous parts of the world, and in those areas is often used as a commercial source of tannin or a source of firewood for local communities. In areas where it has been introduced, it is often considered a weed, and is seen as threatening native habitats by competing with indigenous vegetation, replacing grass communities, reducing native biodiversity and increasing water loss from riparian zones.
In its native range A. mearnsii is a tree of tall woodland and forests in subtropical and warm temperate regions. In Africa the species grows in disturbed areas, range/grasslands, riparian zones, urban areas, water courses, and mesic habitats at an altitude of between 600-1700m. In Africa it grows in a range of climates including warm temperate dry climates and moist tropical climates. A. mearnsii is reported to tolerate an annual precipitation of between 66 to 228 centimetres (26 to 90 in), an annual mean temperature of 14.7 to 27.8 °C (58.5 to 82.0 °F), and a pH of 5.0 – 7.2. A. mearnsii does not grow well on very dry and poor soils.