Mr Johns worked as an Examiner in the British Patent Office in London and there read up on his boyhood interest in evolutionary theory. He used a wide range of books on biology and related subjects, obtained from various London libraries.
When he retired in 1973 he began to buy copies of Darwin's books from second hand shops and dealers. He discovered Freeman and Barrett's bibliography The Works of Charles Darwin, and extended his collection by buying works by other figures who played a role in the history of evolutionary theory, such as Erasmus Darwin, the theologian William Paley, Charles Lyell, Thomas Huxley, and Alfred Russell Wallace.
When he was in his 90s, and no longer able to enjoy his books due to failing eyesight, Mr Johns offered the collection to the University of Kent's Darwin College. His books are now housed in the Templeman Library. Darwin College retained a portrait of Darwin by John Collier. Jack Johns died at the end of 2009.
The Templeman Library mounted an exhibition to coincide with the bicentenary of Darwin's birth on 12 February 1809, illustrating the comprehensive nature of the collection.