February 8 – Albert Spalding of the Chicago White Stockings meets with star player Mike "King" Kelly for contract talks. Kelly wants his $375 bonus for good behavior during the 1886 season. Spalding not only refuses the bonus, but also refuses to refund $225 in fines levied against Kelly for drinking. Spalding has already sold all 3 starting outfielders from the defending champion White Stockings and is aggressively looking to rid his team of drinkers.
February 16 – Mike "King" Kelly is sold to the Boston Beaneaters for $10,000, more than double the price ever paid for any player. Kelly becomes commonly known during that time as "$10,000 Kelly" because of the sale.
February 20 – John Montgomery Ward, president of The Brotherhood of Professional Baseball Players speaks out against the increasing pattern of player-selling. The Brotherhood will later start the rival Players League in 1890.
May 6 – The National Colored Base Ball League begins with the New York Gorhams beating the Pittsburgh Keystones by a score of 11-8. The league would fold on May 23 after having played only 13 official league games.
May 23 – National Colored Base Ball League or the League of Colored Baseball Clubs, the first attempt to have a league consisting of all-black teams, folds after two weeks in operation.
May 24 – Jerry Denny of the Indianapolis Hoosiers gives Mike "King" Kelly a dose of his own medicine when he prevents Kelly from scoring by grabbing his belt and holding him at third base as Kelly attempts to tag up on a fly ball. The umpire who was watching the flight of the ball, didn't see Denny's action. Kelly was well known in baseball for bending the rules to his advantage when the umpire was not looking.
May 28 – Pitcher Tony Mullane, suspended by the Cincinnati Red Stockings, sues the team for lost pay. Mullane and the Reds will eventually settle the suit out of court and Mullane will return to the team in mid-June.
June 27 – Rookie pitcher George Van Haltren makes his major league debut for the Chicago White Stockings and ties a major league record by issuing 16 walks in a losing cause. Van Haltren will soon move to the outfield and amass over 2500 hits in a 17-year career.
July 22 – A pitcher named Frank Chapman makes his only appearance for the Philadelphia Athletics. This appearance is notable because he would later be misidentified as a 14-year-old named Fred Chapman, who was long believed to be the youngest player in major league history. This error would not be discovered until 2009.
August 23 – Ned Williamson hits a mammoth home run over the center field fence at Boston's South End Grounds. It is only the 2nd ball hit over the fence in center field in the park's 17-year history. Rain later washes out the game and Williamson's home run with it.
August 29 – Denny Lyons is held hitless for the first time since May 23, a span covering 52 games. Under the rules of 1887, a walk counted as a base hit, and as a result Lyons' feat is largely ignored today since he needed walks to continue his streak on July 22 and again on August 19.
September 11 – The St. Louis Browns players refuse to play an exhibition game against the Cuban Giants. In a letter to owner Chris von der Ahe, the players wrote "we will cheerfully play against white people at any time and think that by refusing to play [blacks] we are only doing what is right."
October 9 – Guy Hecker, star pitcher and hitter of the Louisville Colonels who plays other positions when not pitching, sets a defensive record for first basemen by recording zero fielding chances in a 9-inning game.
October 21 – After a rainout the day before, the Browns pull off a triple play in an 11-4 morning victory over Detroit.
October 21 – The Detroit Wolverines win the series with a 13-3 afternoon win over the St. Louis Browns. Even though the Wolverines have won the series, the remaining 4 games will be played as they have previously been scheduled in various cities.
October 26 – The Browns win the final game of the series but Detroit wins the series 10 games to 5.
October 27 – The Brotherhood of Professional Base-Ball Players agree to not sign contracts until an agreement has been reached with club owners regarding salary caps and the reserve rule.
November 2 – The Philadelphia Athletics of the American Association are sold to a syndicate headed by Henry C. Pennypacker. The three longtime partners, Sharsig, Simmons, and Mason, still hold a sizable block of stock.
November 14 – The Cleveland Blues announce new uniforms for the 1888 season. The web-like pattern on the uniform will inspire the nickname "Spiders" which the club officially adopts.
November 16 – The joint rules committee drops the 4-strike experiment from 1887 and returns to the standard 3-strike rule. The committee also drops the base on balls as counting for a hit in official statistics.
December 2 – The International League disbands, as the Syracuse, Toronto, Hamilton and Buffalo teams split off to form the International Association, while Newark, Jersey City, Wilkes-Barre and Scranton, become the nucleus of the Central League.
December 8 – The American Association agrees to increase ticket prices to .50¢ for the 1888 season. The AA will revert to the original .25¢ fee in August after suffering attendance and revenue losses through the season.