After naval service in the Civil War, Zettlein returned to Brooklyn, where he starred for the Atlantics, one of the top teams of the day. He was the winning pitcher in 1870 when the Atlantics ended the Cincinnati Red Stockings’ undefeated streak. With the formation of the National Association, he joined the Chicago team that seemed certain of the first pennant until its ballpark burned down during the Chicago Fire. Zettlein won 125 NA games in five years but slumped to 4-20 in 1876, his only NL year.