Officially, he was 5 feet 7 inches (1.70 m) in height and weighed 144 pounds (65 kg).

Slagle began his professional career in minor league baseball in 1895.

He played one season in Washington, D. C. before signing with the Philadelphia Phillies when the Senators folded.

He was the Cubs' starting center fielder for three of their National League championships, from 1906 to 1908, which included two World Series victories.

Slagle became the first player to successfully accomplish a straight steal of home in World Series play.