Pfeffer helped the Robins win the 1916 and 1920 National League pennants.

On March 25, 1920, in the top of the first inning during an exhibition game against the New York Yankees, Pfeffer threw a pitch that struck Yankees second baseman Chick Fewster in the temple, knocking him unconscious for approximately ten minutes.

Fewster suffered a fractured skull and concussion, and temporarily lost his ability to speak.

As of the end of the 2014 season, Pfeffer ranked 96th on the MLB career ERA list (2.77)[3] and tied for 73rd on the MLB career hit batsmen list (105).

[5] Pfeffer died in Chicago at the age of 84, and is interred at Rock Island National Cemetery.