| John Mercer | |
| Position: | First baseman |
| Bats: | Left |
| Throws: | Left |
| Birth Date: | January 22, 1892 |
| Birth Place: | Taylortown, Louisiana |
| Death Place: | Shreveport, Louisiana |
| Debutleague: | MLB |
| Debutdate: | June 25 |
| Debutteam: | St. Louis Cardinals |
| Finalleague: | MLB |
| Finaldate: | June 25 |
| Finalteam: | St. Louis Cardinals |
| Statleague: | MLB |
| Stat1label: | Games played |
| Stat1value: | 1 |
| Stat2label: | At bats |
| Stat2value: | 1 |
| Stat3label: | Hits |
| Stat3value: | 0 |
| Teams: | |
John Locke Mercer (January 22, 1892 - December 22, 1982) was a Major League Baseball first baseman. Mercer played in one game for the St. Louis Cardinals on June 25, .