On March 17, 2010, Jon Heyman of Sports Illustrated reported that Washington tested positive for cocaine during the 2009 season and has acknowledged using cocaine.
In 2010, Washington became the second manager of the Rangers franchise (after Johnny Oates) to take his team to the postseason. On October 12, 2010, Washington became the first manager in franchise history to win a playoff series, with a 3–2 victory in the ALDS over the Tampa Bay Rays. On October 22, 2010, Washington's Rangers defeated the New York Yankees in the ALCS in six games, to advance to their first World Series in franchise history, before losing to the San Francisco Giants in five games. He also became the third African American to manage a team into a World Series, joining Cito Gaston, who managed the Toronto Blue Jays to the World Championship in the 1992 and 1993 World Series, and Dusty Baker, who managed the Giants in the 2002 World Series.