This was the first World Series appearance for the franchise that began its existence as the Montreal Expos in 1969, and moved to Washington in 2005 as the Nationals. The Nationals were also the last team from the 1969 expansion class (which also included the Kansas City Royals, Milwaukee Brewers - who began as the Seattle Pilots - and San Diego Padres) to earn a trip to the series. The World Series appearance also means that all National League teams have been to the World Series at least once. The only American League team not to reach the Fall Classic is the Seattle Mariners, who were part of the 1977 expansion. The Astros and Nationals had never met in the postseason before, despite Houston's stint in the National League from 1962-2012. The Astros and Nationals did not play an interleague game in 2019, and last faced each other during the 2017 regular season. The two teams share a Spring training site in West Palm Beach, Florida, and opened the 2019 Spring training schedule against each other. This was the second World Series to feature two expansion teams, the first being in 2015 between the Kansas City Royals and New York Mets.[note 2]
The Nationals had an 82–80 (.506) win-loss record in 2018, and started the 2019 season with a 19–31 (.380) record. Second-year manager Dave Martinez began to receive public pressure to be fired by the Nationals. The team engineered a turnaround and finished the season in second place in the National League East, four games behind the Atlanta Braves, ending the year with a 93–69 (.574) record. The Nationals were one of two teams to qualify for the playoffs as a wild card team from the National League. Martinez had missed three games in September due to a cardiac catheterization procedure to treat angina.
The Nationals defeated the Milwaukee Brewers at home in the National League Wild Card Game, coming behind from a 3–1 deficit in the eight inning to win 4–3. The Nationals then defeated the Los Angeles Dodgers, who had won the previous two National League pennants, in the National League Division Series. The Nationals were behind two games to one, and won their second and third elimination games of the postseason to take the best-of-five series. The postseason series win was the first in Washington Nationals history.[note 3] In the National League Championship Series, the Nationals swept the St. Louis Cardinals (who had defeated the Braves in the Division Series round) in four games to secure the first pennant in franchise history (including their time as the Montreal Expos from 1969 to 2004). It was the first World Series appearance for a Washington, D.C., team since 1933. The franchise known as the Washington Senators were a charter franchise of the American league who played in D.C. from 1901–1960 before moving to Minnesota to become the Twins. The Senators' name was passed to an expansion franchise that began play the year following year, 1961, and that team played in D.C. through 1971 before moving to Texas as the Rangers. There was no major league baseball team in Washington, D.C. between 1972–2004.
The Astros won the 2017 World Series, the franchises' first World Series championship. They lost in the 2018 American League Championship Series to the Boston Red Sox. The Astros finished the 2019 regular season with a 107–55 (.660) win-loss record, which was the best in baseball. They won the American League West.
The Astros played the Tampa Bay Rays, who won the American League Wild Card Game, in the American League Division Series. Houston defeated the Rays in five games. In the American League Championship Series, the Astros defeated the New York Yankees in six games. This was Houston's third World Series appearance and second in three years.