Cassell and Randy Wittman, the former head coach of Cleveland and Minnesota, were hired as assistants to new Wizards coach Flip Saunders.
"After 15 seasons playing in this league, I have accomplished all that I have dreamed of as a player," Cassell said. "Now the time has come for to me to take my love for the game to the coaching ranks and pass on what I've learned. This team is loaded with talent, and it's a great way for me to start my coaching career."
Cassell played in 993 regular season games for eight teams, averaging 15.7 points and 6.0 assists. He won championships with the Houston Rockets in 1994 and 1995 and the Boston Celtics in 2008. He made his only appearance in the All-Star game in 2004, the same season he reached the Western Conference finals with the Timberwolves under Saunders.
His former coach had been discussing the job with Cassell even before he accepted the Wizards job in April.
"I always regarded Sam as a coach on the floor during his playing days," Saunders said. "And he brings instant credibility as a three-time NBA champion and a proven winner."
Wittman and Cassell are the replacements for Randy Ayers and Phil Hubbard, who were assistants under fired coach Eddie Jordan and interim coach Ed Tapscott. Assistant coach Wes Unseld Jr. remains on the staff, while Dave Hopla is not expected to return.
Wittman coached the Timberwolves for nearly two seasons before he was fired in December 2008. He coached the Cavaliers from 1999-2001 and has been an assistant with three other teams.