Duffy entered the National League with the Chicago White Stockings in 1888 and shortly thereafter earned the reputation of an outstanding outfielder and powerful hitter. He switched leagues, joining the American Association's Boston Reds in 1891; he then returned to the NL with the Boston Beaneaters in 1892, where he enjoyed his best seasons. Playing in Boston from 1891 until 1900, Duffy knocked in 100 runs or more 8 times. In 1894 Duffy had one of the greatest seasons in baseball history, leading the league with 18 home runs, with 145 RBI and a .440 batting average (see Triple crown). Duffy's .440 average is the Major League single season batting average record. It was originally listed as .438, but modern researchers for the Society for American Baseball Research (SABR) determined it was