Flack’s .974 lifetime fielding average was the best ever for a right fielder when he retired, and was only a point behind the record for all outfielders. He made it to the majors with the Chicago Whales of the Federal League in 1914-15, stealing 37 bases each year. In 1915, when the Whales were league champions, that total ranked fourth in the league, and Flack’s career-high .314 was fifth-best. The Cubs bought out the Whales after the FL folded. Flack spent the next six seasons as the Cubs’ regular leadoff hitter, scoring 85 runs in 1920 for his personal NL high. He twice led NL outfielders in fielding, with very high averages for the time: .991 (1916) and .989 (1921).
When the Cubs lost the 1918 World Series, it was Flack’s error in the third inning of the final game that let in the Red Sox’ only two runs. Contrary to his record as listed in Macmillan’s Baseball Encyclopedia, he did not have an RBI in the Series.