In 1860, James N. Kerns formed a club, simply named "Athletic Base Ball Club", that soon dominated amateur play in the area (Jordan 1999). Harper's Weekly chronicled a match between Athletic and Atlantic of Brooklyn for the baseball championship in 1866. A famous Harper's illustration shows the Athletic players in uniforms with the familiar blackletter "A" on front.
When newspapers developed stand-alone game scores and league standings, the club was termed Athletic (Base Ball Club being dropped in any case). In prose the team was commonly called the Athletics, plural, and later generations have usually called both club and team the "Philadelphia Athletics".
The Athletics turned professional in the late 1860s and helped establish the first league, National Association of Professional Base Ball Players (NA), which began play in 1871. Their home field had been at 15th and Columbia, an otherwise unnamed venue informally called "the Athletic grounds." For 1871 they relocated to Jefferson Street Grounds, playing most of their home games there until being expelled from the major leagues after the 1876 season.
The Athletics were one of the most successful National Association teams, winning the first pennant with a record of 21 wins and 7 losses (.750), two games ahead of the Boston Red Stockings and Chicago White Stockings. Actually, the race was much closer: the primary official criterion then was neither games nor winning percentage, but wins, and the three clubs finished in the order given with 21, 20, and 19 victories. The final game of the season, played on October 30 in Brooklyn, saw Athletic defeat Chicago, 4–1, clinching the title. Chicago had become a road team following the Great Chicago Fire. (Nate Berkenstock, a 40-year-old amateur who played right field for Philadelphia that day due to injuries, made his only big-league appearance in that game.)
While Boston dominated the NA, winning the other four pennants, the Athletics and New York Mutuals also fielded teams every year, with Philadelphia winning a few more games overall but never challenging Boston.
Dick McBride served as regular pitcher for more than a decade and as captain throughout the NA seasons, which gives him manager credit today. Other star players include Al Reach in the 1860s and Cap Anson, who played from 1872 to 1875 (Anson took over as captain near the very end of the 1875 season).
The Athletics also played one game in Dover, Delaware on June 24, 1875. They played at Fairview Park Fair Grounds.
During their five-year existence the Athletics won 165 games and lost only 86 for a winning percentage of .657. Notable players on their roster included Hall of Famer Cap Anson, infielder Ezra Sutton, and pitcher/manager Dick McBride.