He married Evelyn Hall in Sydney in 1929. The couple had four daughters.
Earnshaw set up a business, Reilax Engineering, which specialised in production-line problems. The business prospered for over 30 years in North Sydney, New South Wales, until he retired in 1969. He invented a number of machines for filling metal and glass containers for the food, cosmetics, chemical and pharmaceutical manufacturing industries. For the Red Cross Blood Bank, Earnshaw devised an oscillator for separating plasma from blood. He was a pioneer of industrial chromium plating in Sydney. His oil-free compressor was widely used on tractors, graders, road rollers and similar earthmoving equipment.
In 1944, Earnshaw was a co-founder of the Book Collectors Society of Australia, and a lifelong supporter of the society. He owned a large collection of books specialising in Australian history and literature. Most were sold at auction in 1979.
^Russell, Eric (1983). The modest engineer. Biblionews, 8(1), p. 12–15.
^Edmonds, Ross (September 2021). "John William Earnshaw and his contribution to Australian historical studies". Biblionews and Australian notes & queries (411): 91.
^Earnshaw, John (1959). Thomas Muir: Scottish martyr [Studies in Australian and Pacific History, no 1 (W. W. Stone, editor)]. Cremorne, NSW: Stone Copying Company.
^Earnshaw, John (1971). Early Sydney cabinetmakers 1804 – 1870: A directory with an introductory survey. Sydney: Wentworth Books. ISBN0855870028
Earnshaw, John (1959). Thomas Muir: Scottish martyr [Studies in Australian and Pacific History, no 1 (W. W. Stone, editor)]. Cremorne, NSW: Stone Copying Company.
Earnshaw, John (1971). Early Sydney cabinetmakers 1804 – 1870: A directory with an introductory survey. Sydney: Wentworth Books. ISBN0855870028.