Established on August 14, 1855, Atlantic became a founding member of the National Association of Base Ball Players, the amateur sport's first governing body, in 1857. (There were no professional clubs at the time.) In 1859, with a record of 11 wins and 1 loss, Atlantic emerged as the recognized champions of baseball. Atlantic held the championship through the 1861 season, albeit in controversial fashion. In a third and deciding game with Excelsior of Brooklyn, Excelsior was leading 8–6 and had men on base, but was forced to withdraw by a rowdy crowd of Atlantic partisans and gamblers. The game was declared a draw, and the championship retained by Atlantic.
Atlantic held the championship again through the 1861 season, which was shortened due to the American Civil War, before finally surrendering it to archrival Eckford of Brooklyn in 1862. Atlantic recaptured the pennant in 1866 with a season record of twenty wins, no defeats, and a single tie as the only blemish on its record.
Atlantic went undefeated in 1865 with an 18–0 record, sweeping series against chief rivals Mutual of New York and Athletic of Philadelphia. Great players of this era included Joe Start, Dickey Pearce, Charlie Smith, Fred Crane, and Tom Pratt. Atlantic's 36-game winning streak was finally broken in June, 1866 by Irvington, NJ. Atlantic retained the pennant that year by splitting a two-game series with Athletic of Philadelphia and declining to schedule a series with Union of Morrisania. Atlantic did surrender the title to Union in 1867.
When Atlantic defeated Eckford to regain the pennant in 1869, Atlantic had already lost to the Cincinnati Red Stockings. This allowed Atlantic to claim the championship over the undefeated Cincinnati club under the "challenge" format of the National Association of Base Ball Players, which resembled modern boxing championship rules rather than a league or tournament format. This outcome undoubtedly contributed to the tremendous anticipation when Cincinnati came to Brooklyn with an 89-game winning streak to meet the Atlantics on June 14, 1870 at Atlantic's home Capitoline Grounds. An estimated crowd of fifteen thousand paid 50 cents a piece to see Atlantic win 8–7 in extra innings in one of the most significant games in baseball history. Atlantic surrendered the title later in the year, though, to Mutual.