Johns was born in Cornwall, England around 1826. His father Thomas Johns was a blacksmith. He might have had smallpox in his youth, as later his face is said to be "pockmarked". After his father died, Johns and his three brothers worked in the copper mines. In 1841 the family was living at Illogan, Penwith, Cornwall. By 1848 Johns had moved to Wales, working as an iron ore miner, at the Clydach Iron Works.