He appeared in 169 games across four seasons in Major League Baseball, principally as a shortstop, for the Buffalo Bisons (1880), Detroit Wolverines (1881), Cleveland Blues (1881, 1884), and Philadelphia Athletics (1883-1884).

He was the starting shortstop, and with a .310 batting average the leading hitter, for the 1883 Athletics team that won the American Association pennant with a 66-32 record.

[1] Moynahan began his career as a professional baseball player in 1879 with the Davenport Brown Stockings of the Northwestern League.

Moynahan appeared in 95 games, all at shortstop, for the Athletics during the 1883 season, compiling a .310 batting average, scoring 90 runs, and hitting 18 doubles and 10 triples with 67 RBIs.

[1] He was the starting shortstop, and the leading hitter, for the 1883 Athletics team that won the American Association pennant with a 66-32 record.