Chris von der Ahe of the St. Louis Browns purchases 19-year-old Silver King from the National League, who hold the rights to all of the 1886 Kansas City Cowboy players after the NL bought out the franchise. King will go on to win 111 games in the next 3 seasons for the Browns, leading them to 2 American Association pennants.
April 20 – Albert Spalding of the Chicago White Stockings completes his purge of players who drink by trading holdout pitcher Jim McCormick to the Pittsburgh Alleghenys for rookie George Van Haltren and $2,000 cash.
April 22 – Tony Mullane of the Cincinnati Red Stockings pitches his first game in the state of Missouri since 1883 after resolving the injunction he had been under for nearly 4 years. Mullane beats the St. Louis Browns 5-2.
April 30 – Tip O'Neill outfielder for the St. Louis Browns hits for the cycle in a 28-11 drubbing of the Cleveland Blues.
May 2 – Fred Carroll of the Pittsburgh Alleghenys hits for the cycle in an 8-3 win over the Detroit Wolverines.
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 7 – Tip O'Neill of the St. Louis Browns hits for the cycle for the 2nd time in 5 games. The Browns beat the Louisville Colonels, 12-7.
May 9 – Ed Morris of the Pittsburgh Alleghenys refuses to pitch his scheduled start and is suspended by the club for 3 weeks.
May 14 – John Roach of the New York Giants switch-pitches in a 17-2 loss to the Philadelphia Quakers in his only major league appearance.
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.
May 30 – New York Giants pitcher Bill George sets a major league record by walking 16 batters in a 12-11 loss to the Chicago White Stockings.
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 1 – The Philadelphia Quakers and the Detroit Wolverines set a record by scoring in 15 of the 18 half-innings in the Wolverines 17-13 victory.
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 batter, 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.
July 26 – Philadelphia Athletics catcher John Milligan sets a record by recording 3 assists in one inning.
August 9 – Charlie Buffinton of the Philadelphia Quakers pitches his 2nd consecutive 1-hitter.
August 10 – For the second time in his career, first-baseman Dave Orr of the New York Metropolitans hits for the cycle. Despite Orr's efforts New York loses to the Baltimore Orioles, 6-5.
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 26 – Cincinnati Red Stockings second baseman Bid McPhee hits for the cycle against the Baltimore Orioles. Cincinnati loses, however, 19-11.
August 27 – Mike "King" Kelly and Ezra Sutton set a record for most runs scored in a game by 2 teammates as they score 6 times each in the Boston Beaneaters 28-14 win.
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.
August 31 – The New York Metropolitans become the first team ever to use 5 pitchers in one game in a 25-11 loss to the Louisville Colonels.
September 3 – The St. Louis Browns clinch their 3rd straight American Association pennant with a 7-4 winover the New York Metropolitans.
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."
September 28 – The Detroit Wolverines win the National League pennant with a 7-3 victory over the Indianapolis Hoosiers.
October 8 – The New York Metropolitans franchise is sold to Brooklyn Grays owner Charlie Byrne for $15,000.
October 8 – The Philadelphia Quakers defeat the New York Giants 6-3 and end the season with 16 wins and 1 tie in their final 17 games to finish in 2nd place in the National League.
October 9 – Tip O'Neill of the St. Louis Browns finishes the season as the American Association leader in doubles, triples and home runs, a feat which has never been duplicated.
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 10 – The St. Louis Browns win the first game of the best of 15 World's Series with a 6-1 win over the Detroit Wolverines.
October 11 – The Wolverines take Game 2 by the score of 5-3.
October 12 – Detroit wins Game 3 in 13 innings 2-1.
October 13 – Lady Baldwin pitches a 2-hitter in leading Detroit to an 8-0 victory.
October 14 – The Browns win Game 5 by the score of 5-2 and now trail in the series 3 games to 2.
October 15 – Detroit wins 9-0 in Game 6.
October 17 – The Wolverines beat St. Louis 3-1 and go up 5 games to 2 in the series.
October 18 – Detroit wins again in Game 8 by the score of 9-2.
October 19 – The Detroit Wolverines increase their series lead to 7 games to 2 with a 4-2 win over the St. Louis Browns.
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 17 – The club owners officially recognize the Brotherhood of Professional Base-Ball Players by meeting with John Montgomery Ward, Ned Hanlon and Dan Brouthers.
November 21 – In the American Association, the St. Louis Browns announce a trade that ships Bill Gleason and Curt Welch to the Philadelphia Athletics in exchange for Fred Mann, Chippy McGarr, and Jocko Milligan, plus $3,000. This is the first of a significant number of trades or sales in the majors, mostly to the Brooklyn Grays.
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.