There was quite a bit of parity in the American League, as 10 teams finished within 10 games of each other, and only 3 teams (Yankees, Orioles, and Indians) had losing records. The standings in the American League West were quite notable because all the teams in that division finished with at least a .500 record.
February 4 – The 12 members of the board of directors of the Hall of Fame vote unanimously to bar Pete Rose from the ballot. He will become eligible again only if the commissioner reinstates him by December 2005.
April 21 – The Chicago Cubs score five runs in the top of the eleventh inning, but the Pittsburgh Pirates comeback with six runs in the bottom of the inning for the victory; the greatest extra-innings comeback (in terms of runs) in Major League history.
July 7 – Outside a restaurant in Arlington, Texas, American League umpire Steve Palermo is shot and paralyzed from the waist down after aiding a woman who was being mugged. The assailant is later sentenced to 75 years in prison.
July 26 – Montreal Expos pitcher Mark Gardner throws a no-hitter through nine innings, but does not complete it when his team fails to score against Los Angeles Dodgers starter Orel Hershiser and reliever Kip Gross. Gardner loses the no-hitter and the game in the tenth inning when the Dodgers get three hits and score the only run of the game. The Expos only get two hits.
August 11 – In only his second Major League game, and first Major League start, Wilson Álvarez throws a no-hitter as the Chicago White Sox beat the Baltimore Orioles, 7–0. It is the fifth no-hitter of the 1991 season, not including Mark Gardner's nine inning no-hitter that was lost in the tenth on July 26.
September 4 – Removing an "asterisk" which was never universally recognized, the Statistical Accuracy Committee decides to put Roger Maris' 61 home run season of 1961 ahead of Babe Ruth's 60 mark of 1927. Regarding the expunging of the asterisk, historian Bill Deane later points out, "It was an easy job: the asterisk never existed. Maris' record was, from 1962 until 1991, listed separately from Ruth's and was never actually defined by 'some distinctive mark.'" The eight-man panel also re-defines a no-hit game as one which ends after nine or more innings with one team failing to get a hit, thereby removing 50 games from the list that had previously been considered hitless, including the 1959 performance of St. Louis Cardinals' Harvey Haddix, who pitched 12 perfect innings against the Milwaukee Braves, and Cincinnati Reds' Jim Maloney 1965 1–0 loss to the New York Mets in 11 innings. Another casualty is Boston Red Sox reliever Ernie Shore 27 straight outs on June 23, 1917, a game in which he relieved Babe Ruth after being ejected for protesting a walk to Ray Morgan, the first Washington Senators batter he faced. Morgan was thrown out trying to steal second, and Shore retired all 26 men he faces in a 4–0 win‚ getting credit in the books for a perfect game.
September 15 – Smokey Burgess, a former major leaguer and previous holder of the record for most pinch-hits, dies at age 64.
September 16 – Otis Nixon, the league's leading base stealer and catalyst on the Atlanta Braves' run from last to first, fails a drug test and is suspended for sixty days, consisting of the rest of the 1991 baseball season and the first six weeks of the 1992 season. The Braves lose the first two games without Nixon but rebound to win the National League pennant.
October 2 – Atlanta Braves pitcher Tom Glavine becomes the first 20-game winner in the majors by beating the Cincinnati Reds. The win assures Glavine of the Cy Young Award when it is given in November.
October 3 – Chicago White Sox catcher Carlton Fisk hits two home runs, including a grand slam, to lead the White Sox to a 13–12 victory over the Minnesota Twins. In doing so, just nine months shy of his 44th birthday, Fisk becomes the oldest 20th-century player to collect a two-HR game. His 7th-inning grand slam off Steve Bedrosian also makes him the oldest major leaguer ever to hit a bases-loaded homer. Cap Anson, at 45, hit two home runs on this date in 1897, and is the oldest major league player to hit a pair.
October 5 – The Atlanta Braves become the second team in two weeks to go from last to first when they beat the Houston Astros, 5–2. Moments later, the San Francisco Giants eliminate their arch-rivals, the Los Angeles Dodgers, when Trevor Wilson pitches a 4–0 complete game shutout, handing the National League West division title to the Braves. John Smoltz gets his fourteenth win of the season as the Braves close out with eight consecutive wins after trailing the Dodgers by two with only ten games left to play.
October 7 – Leo Durocher, who is credited with the phrase 'nice guys finish last,' dies at the age of 86. The same day, the New York Yankees fire Stump Merrill, the ninth major league manager fired in 1991.
October 8 – Despite finishing in second, their lowest finish in his 3½ years as manager, the Boston Red Sox dismiss Joe Morgan and replace him with Butch Hobson. Morgan is the tenth manager fired in 1991.
October 18 – Jim Essian, who replaced Don Zimmer in May, is fired as manager of the Chicago Cubs, the thirteenth and last firing of a manager in 1991. The thirteen firings in a season set a majors record that still stands.