Note: team nicknames are given here according to the modern retroactive convention. According to the 1889 Spalding Official Guide, however, which covered the 1888 season, no nicknames were used for any team - aside from Athletic - with the exception of "Giants" (always within quotation marks).[1]
January 2 – Fred Dunlap signs a contract paying him a $5,000 salary and a $2,000 signing bonus. It is the largest contract ever given to a player to date.
February 2 – The Indianapolis Hoosiers announce that they will have 42 private boxes on top of their new grandstand. The boxes will only be available to season subscribers.
February 13 - The Kansas City Cowboys purchase the contract of Jumbo Davis from the Baltimore Orioles of the International League.
March 2 – The National League reverts to its original policy of a .50¢ admission price with no concessions given to individual clubs.
March 5 – The American Association votes to adopt the use of turnstiles at all parks in order to better control entry into games.
July 4 – Albert Spalding, president of the Chicago White Stockings, has two ticket speculators arrested and jailed after they violate a city ordinance that prohibits the selling of tickets on the street.
July 14 – Adonis Terry tells his teammates that he heard Kansas City Cowboys manager Sam Barkley order substitute umpire Jim Donahue, who was the Cowboys regular catcher, to call a Bridegroom runner out in the 9th inning of a 5–4 game. The Grooms walked off the field in protest‚ forfeiting the game, resulting in a 9–0 score.
July 17 – Tommy McCarthy of the St. Louis Browns goes 5–5 at the plate to go along with 6 stolen bases in a game in which the Browns steal 15 bases in all.
July 28 – Jimmy Ryan of the Chicago White Stockings hits for the cycle and pitches 7 innings in relief in a 21–17 win over the Detroit Wolverines. Ryan becomes the first player to hit for the cycle and pitch in the same game.
August 7 – The American Association votes to allow teams to lower ticket prices to .25¢ and changes the gate guarantee for the visiting team from a percentage to a flat $130.
August 10 – Tim Keefe of the New York Giants wins his 19th consecutive game, setting a new major league record.
August 21 – The Detroit Wolverines blow a 2–0 lead by committing 6 errors in the final 2 innings to lose their 16th game in a row.
August 22 – Silver King of the St. Louis Browns loses a no-hitter in the 9th inning when 2 Browns outfielders let an easy fly ball drop between them.
August 22 – The Indianapolis Hoosiers attempt to play a "night" game at dusk by using natural-gas lights.
August 29 – Joe Quinn makes his first game as a Boston Beaneater a memorable one by homering in the bottom of the 9th to give Boston a 2–1 win over 30 game winner Tim Keefe.
September 4 – Pud Galvin became the first pitcher to win 300 games.
September 6 – The Indianapolis Hoosiers try again to play an evening game with the use of natural-gas lights. Unable to generate adequate lighting to play, the Hoosiers drop the idea for good.
September 12 – The New York Giants are forced to forfeit their game with the Chicago White Stockings when they have no available substitutes to replace the injured Buck Ewing in the 5th inning.
September 20 – Cubs pitcher Frank Dwyer pitches a 3-hit shutout in his Major league debut, defeating the Washington Nationals 11–0 in the first game of a double-header at West Side Park.
October 1 – With darkness looming, the Indianapolis Hoosiers score 3 runs in the top of the 9th inning to take a 4–2 lead over the Washington Nationals when Washington catcher Connie Mack suddenly develops an "injury" to his finger. The delay causes the game to be called because of darkness with the score reverting to the last completed inning, resulting in a 2–1 Washington victory.
October 13 – The National League season comes to a close with the champion New York Giants setting a league attendance record by drawing 305,000 fans for the season.
October 19 – New York takes a 3–1 series advantage with a 6–3 win over St. Louis.
October 20 – The Giants score 5 runs in the bottom of the 8th inning to take a 6–4 victory and a 4–1 lead in the series.
October 22 – New York wins again in Game 6 by a score of 12–5.
October 24 – The Browns get a must-win by scoring 4 in the 8th to beat the Giants 7–5.
October 25 – The New York Giants clinch the series with an 11–3 win over the St. Louis Browns. The final 2 games will be played for revenue purposes with St. Louis winning both contests for an overall series result of 6 games to 4 in favor of the Giants.
November 20 – The Joint Rules Committee reduces the number of balls needed for a walk to 4. With the 4 ball, 3 strike at-bat and overhand pitching rules now in place, baseball in 1889 will be played very similar to the game of today.
November 22 – The National League adopts a five-tier salary structure based on a player's on-field abilities and off-field personal habits, with the salary scale ranging from $1,500–2,500 in each tier. The Brotherhood of the Professional Baseball Players is incensed by the classification system and it will be the impetus for the organization of the Players' League in 1890.
November 23 – The World Champion New York Giants announce the sale of star player and leader of The Brotherhood of Professional Baseball Players, John Montgomery Ward, to the Washington Nationals for $12,000. The deal will fall through after Ward refuses to abide by the sale.
April 29 – Charlie Ferguson, 25, pitcher who won 99 games in his first 4 seasons, including a no-hitter, for the Philadelphia Quakers since 1884; was 30–9 for the 1886 team