Brown signed a four-year, $115 million contract extension with the Celtics. He tied his career-high 34 points this season against the Cleveland Cavaliers on December 28, 2019. In January, he narrowly missed being selected to the 2020 NBA All-Star Game. In the 2020 playoffs, the Celtics advanced to the Eastern Conference Finals for the third time in Brown's four years in the NBA following series victories over the Philadelphia 76ers and Toronto Raptors in four and seven games, respectively. However, Boston was eliminated in those Conference Finals by the Miami Heat in six games.