Harry Vernon Mercer
- Bats Unknown, Throws Unknown
- Height 5' 11½"
- Debut August 2, 1910
- Final Game August 2, 1910
- Born March 10, 1889 in Zanesville, OH USA
- Died June 25, 1945 in Dayton, OH USA
Jack Mercer has the best ERA in the history of major league baseball, since he pitched one inning and gave up no runs for an ERA of 0.00. A teammate, Skip Dowd, also 21 years old, also pitched in just one game for the 1910 Pittsburgh Pirates and gave up no runs, and never appeared again in the majors.
Since Deacon Phillippe and Sam Leever, the Pirates' long-time pitching aces, were both 38 years old, there was some possibility that a spot would be opening up. It was Leever's last year, and Phillippe pitched only 3 more games in 1911. But Mercer and Dowd did not get another chance to show their stuff.