| Jack Mercer | |
| Position: | Pitcher |
| Bats: | Unknown |
| Throws: | Unknown |
| Birth Date: | March 10, 1889 |
| Birth Place: | Zanesville, Ohio |
| Death Place: | Dayton, Ohio |
| Debutleague: | MLB |
| Debutdate: | August 2 |
| Debutyear: | 1910 |
| Debutteam: | Pittsburgh Pirates |
| Finalleague: | MLB |
| Finaldate: | August 2 |
| Finalyear: | 1910 |
| Finalteam: | Pittsburgh Pirates |
| Statleague: | MLB |
| Stat1label: | Earned run average |
| Stat1value: | 0.00 |
| Stat2label: | Record |
| Stat2value: | 0-0 |
| Stat3label: | Strikeouts |
| Stat3value: | 1 |
| Teams: |
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Harry Vernon Mercer (March 10, 1889 – June 25, 1945) professionally Jack Mercer, was a Major League Baseball pitcher who played for the 1910 Pittsburgh Pirates in one game on August 2, 1910. Prior to his brief appearance with the Pirates, he played from 1907 to 1910 in the minor leagues.