Guy Hecker of the Louisville Colonels not only compiled a 26–23 record with a 2.87 Earned Run Average as the Colonels number 2 pitcher, he also won the American Association batting crown with a .341 average. Hecker remains the only pitcher to ever win a batting title.
Matt Kilroy of the Baltimore Orioles throws 4 2-hitters, 3 1-hitters and a no-hitter in his rookie season in 1886. Kilroy also sets the single season major league record with 513 strikeouts.
Jocko Flynn of the Chicago White Stockings goes 23–6 in his rookie season in 1886. Flynn develops arm trouble and never pitches again in the major leagues. His 23 wins are still a record for a pitcher who only pitched in 1 season.
January 4 – St. Louis Browns owner Chris von der Ahe sells the reserve rights of Sam Barkley to the Pittsburgh Alleghenys for $1,000. von der Ahe had previously sold the rights to the Baltimore Orioles on December 24, 1885 but had not received payment from Baltimore. Barkley, in the interim, had already signed a contract with Baltimore. The resulting power struggle within the American Association to resolve the situation would lead to the ouster of league president, Denny McKnight.
February 5 – The lawsuit brought by Fred Thayer and George Wright against Albert Spalding and his Spalding sporting goods company for infringement upon Thayer's patent rights to the catching mask goes to trial. Spalding will be forced to pay royalties to Thayer and Wright when the case is settled.
February 9 – The Kansas City Cowboys are admitted to the National League for a one-year trial. This brings the NL back to 8 teams for 1886.
March 5 – In breaking from official league policy, the National League allows the St. Louis Maroons and the Philadelphia Quakers to drop their ticket prices to .25¢. The removal of the .50¢ requirement for those teams is due to competing teams from the American Association in their respective cities.
March 17 – The Sporting News, founded by Alfred H. Spink, publishes its first issue. It will quickly become the leading source of baseball information in the country.
April 13 – The American Association resolves the Sam Barkley case by allowing Barkley to play for the Pittsburgh Alleghenys and sending Milt Scott from Pittsburgh to the Baltimore Orioles as compensation for losing Barkley. Additionally, the St. Louis Browns are allowed to keep the $1,000 they received from Pittsburgh for Barkley's reserve rights.
April 16 – The final exhibition games between various National League and American Association teams are played with the AA holding a 19–16 advantage over the NL in the games played.
April 22 – The New York Metropolitans open their new 5,000 seat stadium, the St. George Cricket Grounds. The new park boasts illuminated fountains, with an amusement park and restaurants next to it.
April 29 – The New York World prints woodcuts of live action photographs taken during a game.
May 3 – Pat Dealy of the Boston Beaneaters sets a National League record by allowing 10 passed balls, while pitcher Bill Stemmeyer throws 5 wild pitches in a 12–11 loss.
June 4 – Tony Mullane of the Cincinnati Red Stockings gives more ammunition to those suspicious of him throwing games when he gives up 12 runs in the last 2 innings after 7 shutout innings in a 12–7 loss to the Brooklyn Grays.
June 18 – The Cincinnati Enquirer publishes a letter it has received accusing Tony Mullane of throwing games on the Red Stockings previous road trip. When the writer fails to produce evidence after being challenged, Mullane is exonerated by the team.
June 26 – George Stovey makes his professional league debut in the Eastern League with the Jersey City Jerseys.
July 8 – Jumbo McGinnis of the St. Louis Browns, making only his 10th start of the season, shuts out the Baltimore Orioles 10–0. Hours later, McGinnis is sold to the Orioles.
July 9 – Joe Start, one of baseball's original stars, plays in the final game of his career. The 43-year-old veteran played in the original season of the National Association in 1871, although his playing days as an amateur started before the American Civil War.
July 27 – The Brotherhood of Professional Base-Ball Players, led by John Montgomery Ward, publicly announces its existence. The Brotherhood boasts chapters in virtually every major league city. This group will be behind the formation of the Players League in 1890.
July 29 – Tom Ramsey of the Louisville Colonels throws a 1-hitter against the Baltimore Orioles, allowing only a supposed single to Pat O'Connell leading off the first inning. Curiously, though the Baltimore Sun and the Louisville Courier both report the game as a no-hitter, Ramsey is officially credited with a 1-hitter.
July 31 – Tom Ramsey of the Louisville Colonels pitches his 2nd consecutive 1-hitter, striking out 16 in a 2–1 win over the Baltimore Orioles. It also marks the 3rd time in 4 games that the Colonels have 1-hit the Orioles as Dave Foutz had also thrown a 1-hitter against them on July 28.
August 15 – Guy Hecker of the Louisville Colonels pitches a 4-hitter in a 22–5 win over the Baltimore Orioles. More impressively, Hecker gets 6 hits and scores 7 runs in the game which sets a major league record. 3 of Heckers hits are inside the park home runs, another major league record that will be tied in 1897. In addition, it gives Hecker 17 hits in his last 4 games, another major league record that has since been tied. Hecker's 15 total bases for the game also set a record that will be broken in 1894.
August 16 – 1 day after pitcher Guy Hecker's hitting exhibition, St. Louis Brown hurler Bob Caruthers provides his own offensive fireworks by becoming the first pitcher to have 4 extra-base hits in one game by smacking a double, triple and 2 home runs in a game he loses 11–9 after allowing 10 runs in the 8th inning. Caruthers will not only go on to win 30 games in 1886, he will also lead the American Association in slugging percentage and on-base percentage.
September 10 – Dan Brouthers smacks 3 home runs to go along with a single and double and ties the record for 15 total bases in 1 game. Despite Brouthers' heroics, his 2nd place Detroit Wolverines lose to the league leading Chicago White Stockings 14–8.
Pud Galvin walks all 3 batters he faces in one inning in a game against the Brooklyn Grays. Galvin escapes the potential jam by picking all 3 runners off of first base in succession.
September 30 – The Chicago White Stockings accept the challenge issued by the St. Louis Browns to play a World Championship Series. The teams agree on a best-of-7 game format with the winner taking the entire gate money from the series.
October 23 – The St. Louis Browns win the World Championship Series with a 4–3 win over the Chicago White Stockings. Chicago pitcher John Clarkson, pitching his 4th game in 6 days, holds St. Louis hitless for 6 innings as Chicago takes a 3–0 lead. The Browns tie the game with 3 runs in the 8th inning and win it in the bottom of the 10th when Curt Welch scores on a wild pitch in what became known as "the $15,000 slide". The Browns win the entire gate receipts from the series which total $13,920, with each St. Louis player receiving roughly $580 for the championship.
November 16 – The joint rules committee between the National League and the American Association announce the new rules for the 1887 season which include 5 balls for a walk, 4 strikes for an out, the batter calling for pitch location being abolished and establishing a strike zone between the knees and shoulders of the batter. The pitcher can now only take 1 forward step in his pitching delivery and by changing the dimensions of the pitcher's box, the pitching distance is now at 55½ feet.
November 18 – The National League officially admits the Pittsburgh Alleghenys who jump from the American Association. The Alleghenys made a reported profit of $160,000 in 1886 and finished 2nd in the AA, making the decision a fairly easy one for the NL.
December 15 – The American Association adopts a rule that allows a club to reserve a player for as long it wishes, rather than for just one year as had previously been the case.