“Mordecai Brown, of missing digit fame, was too accomplished a moundsman for the New Yorks.” Washington Post, August 7, 1906, p8
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1913. “So many promising moundsmen are grabbed up by the scouts of outside clubs while only a few are ever given trials at home” (San Francisco Bulletin, Feb. 11; Gerald L. Cohen).