On the same day he left the Bulls, he was hired as head coach of the Washington Bullets. The previous coach had been K. C. Jones, who had led them to a 48-34 record and a loss in the Eastern Conference Semifinals to the Cleveland Cavaliers. In his first season, the Bullets went 48-34 while advancing to the Semifinals again after beating the Cavaliers in the First Round, although they lost to the Houston Rockets in six games. The next year was the pinnacle for the team and Motta's career. They went 44-38, but they advanced all the way to the 1978 NBA Finals, where they beat the Seattle SuperSonics in seven games to win the NBA championship. The following year, the team went 54-28 while winning the Atlantic Division. This was not only their sixth division title in eight years, it was also their last division title until 2017. The Bullets went to the 1979 NBA Finals, although they had to fight the full seven games in both the Semifinals and the Conference Finals, nearly blowing a 3-1 series lead to the Atlanta Hawks in the semifinals and having to come back from a 3-1 series deficit from the San Antonio Spurs in the conference finals. In the Finals that year, they played the Seattle SuperSonics once again. The Bullets won Game 1 at home 99–97, but the SuperSonics won the following four games to win the NBA championship. The following year, the Bullets went 39-43, although they qualified for a playoff berth. They were beaten by the Philadelphia 76ers in two games. He resigned as head coach on May 27, 1980.
Motta was the first head coach of the Dallas Mavericks, hired by the team on July 16, 1980. His first team went 15-67, dead last in the league. They did not lose as many games again until 1992 when they lost 60 games. Motta's Mavericks gradually rose up in prominence, rising in finishes in the Midwest Division from 6th in the first season to 4th by the third year. His fourth season (1983-84) was the start of something big for the team, as they went 43-39 while qualifying for the playoffs for the first time. They defeated the Seattle SuperSonics in the First Round to advance to the Semifinals, where they lost to the Los Angeles Lakers in five games. The following year, the Mavs went to the playoffs once again after a 44-38 season, although they lost to the Portland Trail Blazers in the First Round. They went to the Semifinals the following year after a 44-38 year and defeating the Utah Jazz in the First Round, although it ended with another loss to the Lakers. The next season was Motta's last, and it was his best with the team as they went 55-27 and finished 2nd in the Western Conference, winning their first Midwest Division title (the Mavericks did not win a division title again for twenty seasons). However, they lost to the SuperSonics in the First Round in four games. Motta resigned on May 20.