John Jackett was the son of Thomas Jackett, a Falmouth yacht builder. As a youth, John ran away to South Africa and joined the Cape Mounted Police and it was during his time there that he learned to play rugby with the De Beers team. His mother bought him out of the police and he returned to Falmouth to play rugby for his home-town team and also helped out Plymouth and Devonport Albion. He started out as a half-back for Cornwall before switching position to become the finest full-back in the country. He captained the county from 1904 and went on to win 52 Cornwall caps. After a move to Leicester in 1905 he won the first of his 13 England caps against the touring New Zealanders at Crystal Palace in December 1905. The following December he played against the touring South Africans. His last cap was against Scotland at Richmond in March 1909. Jackett captained the Cornwall team that beat Durham at Redruth to win the 1908 County Championship for the fist time; his brother Dick was also in the side.