Thacker was chosen in the first round (fifth overall) of the
1963 NBA draft as a territorial pick by the
Cincinnati Royals. During his three seasons with the Royals, he played about nine minutes per game as a backup guard, averaging 2.8, 2.5, and 3.7 points per game, respectively during the 1963–64 through 1965–66 seasons. On May 1, 1966, he was drafted by the
Chicago Bulls in the NBA expansion draft, but he decided to retire and did not play that season. On August 4, 1967, he returned to the NBA and was signed as a free agent by the
Boston Celtics. It turned out to be a fortuitous trade for Thacker. Averaging about 12 minutes per game and 4.2 points and 2.5 rebounds per game, he earned a championship ring as the Celtics won the NBA title in a six-game finals series over the
Los Angeles Lakers. He also posted a career single-game high of 17 points twice, on February 11, 1968, against the Lakers and again on March 7 against the Bulls. After Thacker's fourth NBA season, on May 6, 1968, he was drafted by the
Milwaukee Bucks in the NBA expansion draft. in six games over the
Los Angeles Stars. In his third and final ABA season of 1970–71, he played eight games and his seven-year professional career ended. ==Coaching career==