When Longboat was a child, a Mohawk () resident of the reserve, Bill Davis, who in 1901 finished second in the Boston Marathon, interested him in running races. He began racing in 1905, finishing second in the Victoria Day race at Caledonia, Ontario. His first important victory was in the Around the Bay Road Race in Hamilton, Ontario in 1906, which he won by three minutes. In 1907 he won the Boston Marathon in a record time of 2:24:24 over the old 24-1/2 mile course, four minutes and 59 seconds faster than any of the previous ten winners of the event. He collapsed, however, in the 1908 Olympic Games marathon, along with several other leading runners, and a rematch was organized the same year at Madison Square Garden in New York City. Longboat won this race, turned professional, and in 1909 at the same venue won the title of Professional Champion of the World by defeating Dorando Pietri and Alfred Shrubb in front of sell-out crowds.