January 28 – New York Supreme Court Justice Morgan J. O'Brien rules in favor of John Montgomery Ward's Reserve Clause case, and by extension the Players' League, by ruling baseball contracts lacked mutuality and were therefore unenforceable. This is the first in several rulings that allows the Players' League to proceed as planned.
February 1 – The National League finalizes its schedule for 1890, but refuses to release it. Speculation abounds that they are waiting for the Players' League to release their own schedule so that the new circuit may purposely schedule conflicting games in the same cities where both leagues have teams.
February 20 – Sam Rice is born in Morocco, Indiana. A quick outfielder with a great arm, Rice will lead the American League in hits twice, in stolen bases once, and collect at least 200 hits on six occasions, while finishing in the top ten in batting average eight times. Rice will gain election to the Hall of Fame in 1963.
February 24 – An anonymous group allegedly offers $1 million to purchase the entire National League. The National League, believing the offer a hoax, turns it down. Some believe the offer was made by the Players' League, knowing the new circuit would refuse the offer, so they could point to the refusal as proof that the National League was in much better financial shape than they claimed.
April 17 – The Players' League is officially launched even though the structure has been in place for several months. Due to player contract wording, the PL's legal representation thought is best to wait until 1890 to officially form. The Players' League had also decided to wait until several lawsuits and injunctions were decided.
April 19 – Amid much hoopla, the National League and the Players' League both open their seasons with directly conflicting games. Both leagues will inflate attendance figures all season in an effort to influence public opinion.
May 1 – George Pinkney of the Brooklyn Bridegrooms has his consecutive games streak stopped at 577 after being spiked in a game. Pinkney's game streak would stand until 1920 when it was broken by Everett Scott but Pinkney's consecutive innings streak, however, would last until it was broken by Cal Ripken, Jr. nearly 100 years later.
May 8 – Wee Willie McGill starts today for the Cleveland Infants against the Buffalo Bisons in a Players' League game. McGill, a virtual infant himself at only 16 years and 6 months old, yields 7 hits, walks 7, strikes out 10, and singles in a 14–5 complete game victory.
May 22 – Harry Wright, manager of the Philadelphia Phillies, is inexplicably struck blind. It will take 10 days before Wright can even distinguish light from dark and he will not return to managing until August 6.
June 7 – Jack McFetridge of the Philadelphia Phillies pitches a 5-hitter in his major league debut, winning 4–1. McFetridge will not appear again in the majors until 1903, when he goes 1–11 for the Phillies.
June 15 – Bill Greenwood of the Rochester Broncos becomes the only left-handed shortstop to participate in a triple play as the Broncos turn it against the Syracuse Stars. It is the last game Greenwood plays at shortstop in his career.
June 21 – Silver King of the Chicago Pirates of the Players' League pitches a no-hitter against the Brooklyn Ward's Wonders but loses 1–0 when the game's only run comes on a 2-base error. As King only pitched eight innings, Brooklyn not having to bat in the bottom of the ninth, this game is not an official major league no-hitter.
July 12 – The visiting Buffalo Bisons pick up a local Brooklyn player named Lewis to pitch in their game against the Brooklyn Ward's Wonders. Lewis allows 20 runs on 13 hits and 7 walks in 3 innings before he is moved to left field for the remainder of the game. It is the only known appearance in the major leagues for Mr. Lewis.
September 6 – In Baltimore, with no umpire present, the Toledo Maumees and the Baltimore Orioles each supply a player to officiate. After 7 innings with the game tied at 2, the Toledo umpire calls the game because of darkness. The Baltimore umpire promptly calls the game a forfeit in favor of the Orioles. The American Association will officially declare the game a tie.
September 17 – The Philadelphia Athletics of the American Association release or sell most of their players because of severe financial problems. The Athletics finish the season losing all 21 of their remaining scheduled games with assorted pick-up players.
September 23 – George Nicol of the St. Louis Browns throws a no-hitter in his major league debut in a game called after seven innings due to darkness. Ed Cartwright chips in with a three-run home run and a grand slam in an 11 run third inning of the 21-2 rout.
November 22 – At the American Association annual meeting in Louisville, the Philadelphia Athletics are expelled for violating the league's constitution. A new team in Philadelphia is admitted, plus entries from Boston, Washington and Cincinnati, replacing Syracuse, Toledo and Rochester.