February 6 – The Providence Grays are accepted into the National League.
February 12 – Fred Thayer, Harvard team manager, receives a patent for the catching mask.
March 8 – Devastated by the gambling scandal from 1877, The Louisville Grays withdraw from the National League after being unable to assemble a competitive team.
April 2 – The National League announces a group of 18 umpires who are eligible to umpire league games. The scheduling of those umpires is left to the individual clubs to decide.
May 1 – In front of 5,500 fans at the brand new Messer Street Grounds, the Providence Grays lose the season opener to the defending champion Boston Red Caps 1-0. Mike "King" Kelly makes his major league debut in the home opener at Cincinnati.
May 9 – Sam Weaver of the Milwaukee Grays pitches a no-hitter against the Indianapolis Blues for Milwaukee's first National League victory. One scorer credited John Clapp with a single, but Weaver is generally recognized today with throwing the no-hitter.
August 14 – "The Only" Nolan is released by the Indianapolis Blues after lying to the team about attending a funeral.
August 19 – In an embarrassing day for the National League, teams from the NL lose 3 of the 4 exhibition games played against non-league teams.
August 25 – Bill Craver is certified eligible for play in the International Association. Craver had been kicked out of the National League after his involvement in a gambling scandal with the Louisville Grays in 1877.
August 31 – Albert Spalding plays in the final game of his career, going 2–4 at the plate while committing 4 errors in the field.
September 12 – The Boston Red Caps clinch the pennant with a 4–2 victory over the Indianapolis Blues.
September 14 – The Indianapolis Blues and Boston Red Caps play an experimental exhibition game in which every pitch thrown is called a ball or strike and only allowing 6 balls before a walk is issued.
September 30 – The National League completes its schedule, marking the first time in professional baseball that a full schedule has been played by all member teams.
October–December
October 2 – The Buffalo Bisons of the International League defeat the National League champion Boston Red Caps behind the pitching of Pud Galvin. The victory over the Red Caps gives Galvin a win over each team in the NL. He finishes the 1878 season with a combined 10–5 record against the NL clubs. Buffalo will become a member of the National League in 1879.
October 3 – The Syracuse Stars defeat Lowell 12-1 and claim the International Association championship. The Stars will join the National League in 1879.
October 8 – The Buffalo Bisons defeat Utica and lay claim to the International Association pennant as well. The league will ultimately award the pennant to Buffalo in early 1879
October 18 – The Indianapolis Blues come up short $2,500 and are unable to account for the money. The players receive $60 each as payment for the remainder of the season. This event signals the end of the Blues.
November 1 – The Boston Red Caps and the Providence Grays play an exhibition game with no outs on foul bounds and 6 balls for a walk to be given.
November 10 – In the Pacific Coast championship game in California, the Athletics defeat the Californias in front of 8,000 fans at the San Francisco National Trotting Park.
December 4 – The National League votes to accept the Buffalo Bisons, Syracuse Stars and the Cleveland Blues for the 1879 season.
December 4 – The National League changes the rules so that now every pitched ball is called a ball or a strike. 9 balls will now result in a walk to the batter. The rules are also changed regarding the batting order. The first batter in a new inning shall now be the next batter in the order after the last batter that ended the previous inning.
December 5 – The National League abolishes all bound outs, including foul balls on 3rd strikes.
December 10 – The Troy Trojans apply for membership to the National League.
December 29 – The Professional Baseball League of Cuba is founded in Havana with Habana defeating Almendares 21-20. The league is known today as the Cuban League and ran continuously until March, 1961, when the Cuban government, ran by Fidel Castro, abolished professional baseball.
