In 1959, backers announced plans for a new “third major league,” the Continental League. Major League Baseball responded by expanding and promising new teams to keep the sport national in scope and to reduce the threat of competition. This set the stage for the first expansion wave of the 1960s.
In late 1960, the American League approved two expansion franchises for the 1961 season: one in Los Angeles (the Angels) and one in Washington, D.C. (a new Senators team). This move helped the AL keep pace with population growth in the West and replaced a franchise leaving Washington.
An expansion draft in December 1960 allowed the Angels and the new Senators to select players left unprotected by existing AL teams. Expansion drafts mattered because new clubs needed credible rosters immediately, even if they were built mostly from veteran and fringe players. This process became a standard tool in later MLB expansions.
The original Washington Senators franchise relocated to Minneapolis–St. Paul for the 1961 season and became the Minnesota Twins. Washington immediately received an expansion replacement club, also called the Senators, keeping MLB in the nation’s capital. The shift highlighted how relocation and expansion were linked in this era.
For 1962, the National League expanded with two new clubs: the New York Mets and the Houston Colt .45s. The Mets helped restore a National League presence in New York after the Dodgers and Giants moved to California, while Houston became a new major-league market in Texas. Together, these teams completed the first four-team expansion push of the early 1960s.
In 1965, Houston rebranded from the Colt .45s to the Astros and began playing in the Astrodome, a pioneering domed stadium. The venue became a symbol of modern, indoor baseball and helped MLB market itself as technologically forward-looking. This period showed how westward growth also brought new stadium designs and new kinds of fan experiences.
The Braves moved from Milwaukee and played their first season in Atlanta in 1966. This relocation brought MLB deeper into the U.S. South and became one of the era’s most visible examples of teams chasing larger or faster-growing markets. It also reinforced a trend: expansion and relocation often followed the same map of postwar population shifts.
After the 1967 season, the Athletics relocated from Kansas City to Oakland for 1968. Kansas City immediately pushed for a replacement club, and the league soon promised expansion as part of the solution. This episode showed how political pressure and threatened lawsuits could shape MLB’s expansion timeline.
In 1969, MLB expanded again: the Royals and Seattle Pilots joined the American League, while the Montreal Expos and San Diego Padres joined the National League. With 12 teams per league, MLB split into divisions (East and West) and introduced the League Championship Series before the World Series. Montreal’s entry also made MLB truly international at the major-league level.
The Seattle Pilots lasted only one season (1969) before financial and stadium problems led to a quick relocation to Milwaukee. They became the Milwaukee Brewers in 1970, an early warning that rapid expansion could produce unstable franchises if ownership and facilities were not ready. MLB soon treated stadium planning and financing as central parts of expansion bids.
In 1977, MLB added two American League expansion teams: the Seattle Mariners and the Toronto Blue Jays. Seattle regained a franchise after the Pilots’ departure, and Toronto strengthened MLB’s presence in Canada beyond Montreal. This expansion helped stabilize MLB’s geographic footprint in the Pacific Northwest and added another major media market.
After more than a decade without new teams, MLB expanded again in 1993 with the Colorado Rockies and Florida Marlins, both placed in the National League. Denver and South Florida were fast-growing markets, and both teams drew strong early fan interest. Their arrival helped set up later league realignment and a new round of expansion interest in the 1990s.
MLB reorganized into three divisions per league for 1994 and planned to add a new postseason format with a “wild card” team. A players’ strike, however, ended the season early and led to the cancellation of the World Series. The disruption delayed the new October structure until 1995, but the plan signaled a shift toward a larger, more playoff-driven league.
In 1995, MLB owners awarded expansion franchises to Phoenix (Arizona Diamondbacks) and the Tampa Bay area (Tampa Bay Devil Rays). These selections reflected MLB’s continued push into high-growth Sun Belt markets and emphasized modern ballparks and regional television potential. The awards set the final major-league expansion of the 20th century in motion.
The Diamondbacks and Devil Rays began play in 1998, bringing MLB to 30 teams. To keep leagues even, the Milwaukee Brewers moved from the American League to the National League, one of the most significant league-alignment changes in modern MLB history. By the end of 1998, MLB had largely completed its 1961–1998 expansion-and-westward-growth story: a nationwide (and partly international) league built around new markets, new stadium demands, and a redesigned postseason structure.
Major League Baseball: Expansion Era and Westward Move (1961–1998)