Armour played professional baseball for several years before he began his career as a manager. In 1891, at age 21, he appeared in 69 games, principally as a right fielder, for the Bradford, Pennsylvania club in the New York–Pennsylvania League. By May 1891, Armour was drawing for his fielding ability. The manager of the Meadville team wrote to the "Sporting Life" that his team might have one at least one game against Bradford "had not that great fielder, Armour, been playing. In the tenth inning of the first game, with one out and Lyons on second, he caught a line hit foul on which he doubled up Lyons and put an end to what, at that stage of the game, looked like a Meadville victory. It was the finest catch on the grounds this season." Armour compiled a .269 batting average and stole 43 bases for Bradford.