Cassell began his career at Florida State University. He was drafted in the NBA Draft 1993, 24 out of the Houston Rockets, whom he later helped to win two NBA championships until he was traded after the 1995-1996 season to the Phoenix Suns. His next stops were the Dallas Mavericks and the New Jersey Nets, during which he, however, had little success.
Two years later he was traded to the Milwaukee Bucks. There he formed, along with Ray Allen one of the best guard duos ever in the history of the NBA. Nevertheless, he was traded in 2003 along with Ervin Johnson in exchange for Joe Smith and Anthony Peeler to the Minnesota Timberwolves. But even there he was traded on 12 August 2005 against Marko Jarić and Lionel Chalmers and a draft pick to the Los Angeles Clippers. On 28 February 2008, he became a free agent until the Boston Celtics secured his services. With the Celtics, he won his third championship in the same year.
Currently, he works as an assistant coach for the Washington Wizards.
- Triple NBA Champion: 1994 and 1995 with the Houston Rockets and 2008 with the Boston Celtics;
- NBA All- Star 2004