Tiernan patrolled the Giants outfield for 13 seasons at a time when players generally flitted from team to team. The low-keyed, lefthanded slugger spent the last half of his career tolerating Giants owner Andrew Freedman, an early baseball tyrant. Tiernan broke in as a left fielder in 1887, batted .287, and even pitched five games in relief. On May 16 he tied a major league record with five outfield errors in a game; on June 15 he tied the NL mark of six runs scored in a game.
Tiernan starred in right field on the 1888-89 New York pennant-winners, leading the league with 147 runs and 96 walks in ’89. In 1891 he won the NL home run crown with 17. He topped the .300 mark seven times, with a high of .369 in 1896. In 1899 he was let go after batting .255 as a part-timer.