The Celtics were one of the original members of the Basketball Association of America, taking the court in 1946 under founding owner Walter Brown and entering the NBA proper after the 1949 merger with the National Basketball League. The franchise was mediocre in its first several seasons until Brown hired Red Auerbach as coach; Auerbach's acquisition of Bill Russell in 1956, paired with returning guard Bob Cousy and a deep supporting cast, launched the most dominant run in North American professional sports history. Boston won eleven championships in thirteen seasons between 1957 and 1969, including eight consecutive titles from 1959 through 1966, with Russell becoming the league's first Black head coach when he took over on the bench for the final two of those championships. After a brief decline following Russell's retirement, the team rebuilt around Dave Cowens, John Havlicek, and Jo Jo White to win again in 1974 and 1976.

A second golden era arrived in the 1980s, when the Celtics drafted Larry Bird and later added Kevin McHale and Robert Parish to form a frontcourt that, together with guards such as Dennis Johnson, produced championships in 1981, 1984, and 1986 under coaches Bill Fitch and K.C. Jones. The franchise cooled considerably through the 1990s, worsened by Bird's 1992 retirement and the sudden death of young star Reggie Lewis in 1993, and endured some of its worst seasons in franchise history before Danny Ainge's front-office rebuild brought in Kevin Garnett and Ray Allen to join Paul Pierce for a title in 2008 under coach Doc Rivers. That core reached the Finals again in 2010 but fell short, and after another rebuilding stretch the Celtics returned to the top of the league in 2024 behind Jayson Tatum and Jaylen Brown under coach Joe Mazzulla, winning an eighteenth championship. In 2025 the team was sold to a group led by Bill Chisholm, ending more than two decades of ownership under the Grousbeck family.