Langford was selected with the 14th overall pick by the Celtics in the 2019 NBA Draft. He went on to appear in 32 of Boston's 72 regular-season games during the pandemic-shortened 2019-20 season, making two starts. Over the course of 11.6 minutes per game, Langford contributed 2.5 points, 1.3 rebounds, 0.3 steals and 0.3 blocks per contest. He also appeared in seven games for the G League Maine Red Claws, averaging 10.6 points, 2.7 rebounds, 1.4 blocks and 1.3 assists per G League contest. In his G League debut, Nov. 9 against Delaware, the Indiana product delivered 27 points, five boards, two dimes and five blocks. On Feb. 5 with the Celtics, the rookie made his first NBA start, posting six points, one rebound, one steal and one block over 28 minutes during a home win over Orlando. Two days later, Langford scored a career-high 16 points with five boards, three blocks and a steal in a home win over Atlanta. Boston was one of 22 teams invited to the Orlando bubble for seeding and subsequent playoff games. Langford appeared in five of Boston's eight seeding games, averaging 2.0 points over 14.4 minutes per game. The rookie also played in seven playoff games before being shut down due to wrist and hip injuries. In Boston's first-round Game 2 win over Philadelphia, Langford chipped in six points, a rebound and a steal in 23 minutes.