St. Louis Browns left fielder Tip O'Neill led the AA in batting average (.435), on-base percentage (.490), slugging percentage (.691), adjusted OPS+ (213), hits (225), home runs (14), total bases (357), runs scored (167), and runs batted in (123).[1][2]
Baltimore Orioles pitcher Matt Kilroy had a win–loss record of 46–19 and led the AA in innings pitched (589.1), wins (46), and shutouts (6). He was second in the AA in earned run average (3.07) and strikeouts (217). He was fourth in the AA in adjusted ERA+ (133).[3][4]
January 18 – The Kansas City Cowboys are admitted to the Western League. Though this incarnation of the franchise folds after one season, three other teams based in Kansas City would also use the Cowboys moniker, the last appearing in the Union Association in 1889.
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 14 – James Billings, an owner of the Boston Beaneaters, reaches a contract agreement with Mike "King" Kelly that will pay him $2,000 per season as well as a $3,000 bonus if the Beaneaters can purchase his reserve rights from the Chicago White Stockings.
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 4 – During the pre-game warm up, Joe Hornung places a bat on home plate and challenges pitcher Charlie Ferguson to hit it, which Ferguson promptly does on his first attempt.
June 9 – New York Metropolitans outfielder Candy Nelson sets a record by starting 3 double plays in one game.
June 16 – Curt Welch of the St. Louis Browns is arrested for assault in Baltimore after breaking up a double play against the Orioles. Welch will be fined $4.50 in court the next day.
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 6 – Alex McKinnon of the Pittsburgh Alleghenys leaves the team after contracting what is believed to be typhoid fever. McKinnon died on July 24.
July 10 – Enforcing a new law, St. Louis police stop the game and arrest Browns owner Chris von der Ahe for conducting business on a Sunday. Within a week a judge will exempt baseball from the law.
July 14 – In an exhibition game against Newark from the International League, Chicago White Stockings player-manager Cap Anson refuses to play if Newark uses their black players, George Stovey and Moses Fleetwood Walker. Newark gives in and benches the 2 players, then beats the White Stockings 9–4.
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.
November 24 – George Hancock invents an indoor baseball game that would become known as softball in Chicago.
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.
July 8 – Frank McIntyre, 27, pitcher who appeared in 3 games in 1883.
July 24 – Alex McKinnon, 30, first baseman who was batting .340 in his first season with the Pittsburgh Alleghenys; previously with New York and St. Louis teams.
November 9 – Billy Riley, 32?, outfielder who played in parts of 2 seasons.
