In the late 1860s and early 70s, Pittsburgh was home to three local amateur baseball teams—the Enterprise Club, the Xanthas, and the Olympics—which played most of their games Union Park.

[2] Allegheny lasted for three years, playing mostly other squads from the northeast within the International Association, but occasionally taking on National League teams in exhibition games.

One long-remembered match took place in early May 1877, when Pud Galvin threw a shutout and hit a home run in a 1–0 victory over the Boston Red Stockings.

In 1887, owner William A. Nimick transferred the club into the National League,[4] and on April 30, the Alleghenies defeated the Chicago White Stockings, 6–2 in front of 10,000 spectators.

[7] Legend has it that, before opening game of the 1887 season, Pittsburgh's catcher Fred Carroll buried his pet monkey beneath home plate.

[4] Also during that season, local businessman Walter Brown organized an early attempt of a league for African American ballplayers, with his Pittsburgh Keystones taking up residence at the park.

[10] Decades later, the Pro Football Hall of Fame discovered a page torn from an 1892 account ledger prepared by Allegheny manager O. D. Thompson with the line item: "Game performance bonus to W. Heffelfinger for playing (cash) $500.

[15] Pirates owner Barney Dreyfuss secured a lease on the park in 1901 to block the upstart American League from establishing a competing baseball franchise in Pittsburgh.

[16] Beyond simply major sporting events, the park often also doubled as a venue for circuses, carnivals, various ethnic and holiday celebrations, and track meets.