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.
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.
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 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 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.