After seven years of proving himself overseas, Wanamaker finally became an NBA rookie during his age-29 season in 2018-19. Wanamaker's body of work in France, Italy, Germany and Turkey convinced the Celtics to sign him to a one-year contract in the summer of 2018. The University of Pittsburgh product ultimately saw action in 36 games with Boston as well as two matches with the G League's Maine Red Claws. With the Celtics, the point guard averaged 3.9 points, 1.1 rebounds and 1.6 assists over 9.5 minutes per game. On Jan. 14, with Kyrie Irving out, Wanamaker played 26 minutes, producing 13 points, four assists, three rebounds and two steals against the Nets. Nine days later, Wanamaker delivered 11 points in a home win over the Cavaliers. Then, during the last game of the season, Wanamaker scored a career-high 17 points to go with seven dimes and six boards in a win over the Wizards. Wanamaker went on to appear in four of the Celtics' nine playoff games. In Game 2 of the second round against Milwaukee, he scored seven points with a steal and an assist in just 7:04 off the bench.