In 1975 Grevey was selected by the Washington Bullets in the first round (18th pick) of the NBA Draft and by the San Diego Sails in the first round (sixth pick) of the 1975 ABA Draft. Grevey signed with the Bullets and played mostly as a backup small forward and shooting guard his first two seasons. When Phil Chenier suffered a season-ending back injury early in the 1977–78 season, Grevey became the starting off guard and averaged 15.5 points per game. The Bullets won their only NBA championship that season, led by Grevey, newly acquired Bob Dandridge and the future Hall-of-Fame duo of Elvin Hayes and Wes Unseld.