March 5 – In what will prove to be one of the more influential off-the-field events in Major League history, representatives of the players elect Marvin Miller to the post of executive director of the Major League Players Association (MLPA).
March 17 – Sandy Koufax and Don Drysdale escalate their threat of retirement by signing movie contracts.
March 30 – Sandy Koufax and Don Drysdale end their 32-day holdout, signing for $130,000 and $105,000 respectively.
April 3 – USC pitcher Tom Seaver signs with the New York Mets. He had been drafted by the Atlanta Braves, but they had signed him to a minor league contract while he was still in college. This voided Seaver's remaining eligibility, and voided the contract. The Mets won a special lottery over Cleveland and Philadelphia to win the right to sign him.
July 3 – Atlanta pitcher Tony Cloninger hits two grand slams in a game against the Giants, the first National League player and first pitcher in history to do so. His nine RBI in a game is a record for pitchers.
September 22 – The Baltimore Orioles beat the host Kansas City Athletics 6–1 to clinch their first American League pennant since moving to Baltimore. Both Brooks Robinson and Frank Robinson have two RBIs. Frank Robinson will end the year as the Triple Crown winner, the first to achieve the feat since Mickey Mantle in 1956. He clinches with a batting average of .316, 49 home runs and 122 RBIs.
September 22 – In a one-game series delayed two days by rain, the New York Yankees lost to the Chicago White Sox 4-1.[22] The game was played in front of just 413 fans in Yankee Stadium I, the smallest crowd in the history of any version of Yankee Stadium and the fifth-smallest crowd in Major League Baseball history. Four days after this game was played (September 26, 1966), broadcast pioneer Red Barber was told that his contract would not be renewed by then-Yankees owner CBS. This has been disputed, but the belief of Barber among others was that his firing was tie to his reporting the small crowd of that infamous game on television [23][24] The game aired locally on WPIX-TV, Channel 11, the Yankees TV home at that time. Barber would never broadcast another game. He was allowed to finish his contract for 1966, however, as what were to be his final three games for the Yankees in Washington against the Senators were rained out and he was not scheduled to work the season-ending series versus the White Sox in Chicago.
October 9 – In Game Four of the World Series, Dave McNally wraps up a brilliant pitching display, and the first World Championship for the Baltimore Orioles, with a four-hit, 1–0 shutout against the Los Angeles Dodgers. Series MVP Frank Robinson hits a home run off Don Drysdale for the only run of the game and gave Baltimore a surprising sweep of the defending World Champion Dodgers. The shutout completes a World Series record 33+2⁄3 scoreless innings pitched by Orioles pitchers, beginning with Moe Drabowsky pitching 62⁄3 innings in relief of McNally in Game One, followed by shutouts by Jim Palmer and Wally Bunker. The Orioles are the last of the original eight American League franchises to win their first World Series.
For the first time, NBC became exclusive national TV broadcaster of MLB. The network replaced ABC as the holder of the Games of the Week package. The New York Yankees and Philadelphia Phillies, which had instead sold their TV rights to CBS in prior seasons, also joined NBC's package. The new package under NBC called for 28 games, as compared to the 123 combined among three networks during the 1960s. NBC also continued to air the All-Star Game and World Series.
