Mauer made one start for the Metropolitans in 1886, and it didn’t go very well. On July 24, he faced 23 batters and got six of them out. It wasn’t entirely his fault; he walked two and allowed nine hits – fielders made 6 errors, but Mauer made a few of them himself. It certainly was MOSTLY his fault. When it was over, 13 runs were scored and eight were deemed earned – a career ERA of 36. Ouch. Chief Roseman came in from the outfield to finish the game… (These are the stats per baseball-reference.com.)

The New York Times noted, “Besides pitching poorly, he made a number of errors (5) and it is thought that his name will be removed from the payroll of the without being given another trial.”

The New York Sun said that rookie pitcher also worked the third inning and was responsible for a 14th run. However, the Sun included details about the new pitcher in the column next to the box score. Mauer was actually playing under an assumed name: Henry Brooks.

In 2014, SABR researchers figured out that Henry Brooks was actually Frederick H. Mauer (c’mon – his middle name is probably Henry, no?) in part because Mauer’s obit noted that he once played professional baseball and other details of his life lined up with articles like the one above. Mauer was born April 19, 1862 in Hamburg, New Jersey and he learned to play ball in New Jersey. After his career ended, he moved to Schenectady, New York, working there as a laborer for more than three decades.

On the other hand, he’s one of the handful of players who died on his birthday (April 18, 1922) in Schenectady.

Notes:

Baseball-Reference.com
Schenectady City Directories
“Mets Beaten Badly,” New York Times, July 25, 1886: 5.
“At Staten Island,” New York Sun, July 25, 1886: 7.
“Base Ball Notes,” New York Sun, July 25, 1886: 7.