1876 - William McLean, from Philadelphia, became the first professional umpire when he umpired the first game in National League history between Boston and Philadelphia on April 22.
1878 - The National League instructed home teams to pay umpires $5 per game.
1879 - National League president William A. Hulbert appointed a group of 20 men from which teams could choose an umpire, therefore becoming baseball's first umpiring staff.
1879 - Umpires were given the authority to impose fines for illegal acts.
1882 - National League umpire Richard Higham became the only major league umpire ever expelled from the game after the League judged him guilty of collusion with gamblers.
1885 - Umpires began wearing chest protectors for the first time.
1901 - Thomas Connolly umpired the first game in the American League between Cleveland and Chicago on April 24.
1903 - Hank O'Day and Thomas Connolly worked the first modern World Series between the Boston Pilgrims and Pittsburgh Pirates.
1906 - William Evans, at 22 years old, became the youngest umpire in major league history.
1909 - The four-umpire system was employed for the first time in the World Series.
1910 - The umpire organizational chart was established. The plate umpire was appointed the umpire-in-chief and the others were field umpires.
1910 - Chicago Cubs manager Frank Chance became the first person ejected from a World Series game when umpire Thomas Connolly threw him out for protesting a home run call.
1911 - Bill Dinneen worked as an umpire in the World Series and became the first person to play and umpire in the Fall Classic. Dinneen played for the Boston Pilgrims in the 1903 Series.
1912 - Both the American and National Leagues had 10 person umpiring staffs with two umpires being used in games and two reserves.
1921 - Umpires in both leagues began the practice of rubbing mud into the balls prior to each game in order to remove the gloss.
1933 - Bill Dinneen, Bill Klem, Bill McGowan and Cy Rigler umpired the first All-Star Game at Comiskey Park in Chicago.
1935 - George Barr of the National League opened the first umpire training school in Hot Springs, Arkansas.
1939 - The Bill McGowan School for Umpires opened.
1941 - Bill Klem, the oldest umpire in major league history at 68, retired from umpiring after working a record 37 seasons and became the National League's first modern chief of umpires.
1946 - Bill McKinley became the first graduate of an umpiring training school to reach the major leagues.
1947 - In the 1947 World Series, featuring the Brooklyn Dodgers and New York Yankees, the current six-man crew was established as an alternate umpire was stationed along each foul line.
1950 - Umpires were no longer allowed to levy fines for illegal acts as that was to be handled by each League president.
1951 - Emmett Ashford became an umpire in the Southwestern International League and became the first black professional umpire.
1952 - The four-man umpiring crew was instituted for all regular season games.
1953 - Thomas Connolly and Bill Klem became the first umpires inducted into the National Baseball Hall of Fame.
1956 - Ed Rommel and Frank Umont broke a long-standing taboo by becoming the first umpires to wear eyeglasses on the field.
1966 - Emmett Ashford became the first black umpire in the major leagues when he reached the American League after 14 seasons in the minor leagues.
1970 - The first strike by umpires in major league history lasted one day during the League Championship Series. This action prompted both the American and National League presidents to recognize the newly-formed Major League Umpires Association and negotiate a labor contract with them.
1972 - Bernice Gera became the first woman to umpire a professional baseball game when she worked a Class A New York-Penn League game.
1973 - Art Williams became the first black umpire to reach the National League staff.
1974 - Armando Rodriguez became the first Hispanic umpire to work in the major leagues as he joined the American League staff.
1979 - Major league umpires went on strike for the third time in history from Opening Day until May 18. Replacement umpires were used during this strike.
1991 - Steve Palermo, an American League umpire, suffered a career-ending gunshot wound while attempting to prevent the robbery of two women.
1996 - National League umpire John McSherry collapsed during an Opening Day game in Cincinnati and passed away after being rushed to a nearby hospital.
1997 - Mike DiMuro became the first American umpire to work a regular season game in the Japanese Leagues.
1998 - Harry and Hunter Wendelstedt became the first father-son umpire combination to work a major league game together.
2000 - On February 24, the World Umpires Association ("WUA") was certified as the exclusive collective bargaining agent for all regular full-time major league umpires.