Dahlen played shortstop for a National League-record 20 years. His specialty was fielding, to which his NL-record 7,500 shortstop assists and ML-record 13,325 chances attest. Yet he lasted long enough to make 972 errors at shortstop, the most by a player at any position in a single league. He was also a consistent hitter with considerable power for the deadball era.
Dahlen broke in as a fleet-footed 21-year-old with the 1891 Chicago Colts (later the Cubs), playing third base and the outfield. By 1895 he was the everyday shortstop. Like many players of the era, he had his best season in 1894 (the year after pitchers were moved back), reaching personal highs of 15 HR, 107 RBI, and a .362 average. He set a record from June 20 through August 6 with a 42-game hitting streak that Willie Keeler exceeded three years later. After failing to hit on August 7, he reeled off a 28-game streak, thus hitting in 70 of 71 games. Twice in his career, in 1896 and 1898, he tripled three times in a game, and in 1900 tripled twice in one inning.
Dahlen played for Brooklyn from 1899 through 1903, then achieved his lifelong dream of playing for the Giants when Brooklyn dealt him for pitcher John Cronin and shortstop Charlie Babb. It was one of the worst trades in Brooklyn history, as Cronin lasted one season, Babb two. Meanwhile, Dahlen led the NL with 80 RBI in 1904, and was the SS for the 1905 World Champion Giants. After a stint with the 1908-09 Braves, he returned to Brooklyn and managed the club for four seasons, never finishing higher than sixth. He was replaced by Wilbert Robinson in November of 1913.