Purnell became a graduate assistant coach at ODU in July 1975, eventually becoming a full-time assistant at the university. During Purnell's tenure as a full-time assistant, he helped ODU reach the postseason seven times (3 NCAAs and 4 NITs). He was selected to the board of directors at the 1998 convention for the National Association of Basketball Coaches (NABC). In 2000, he was appointed by the NABC to serve as a member of USA Basketball Men's Collegiate Committee. That committee is responsible for the selection of collegiate coaches and players for USA Basketball's teams. At the conclusion of that season, Purnell took his team to the championship game of the
NIT, losing to
West Virginia in the final, following wins against
Syracuse,
Air Force, and
Ole Miss. In 2008, he guided the Tigers to a third-place 10–6 record in the
Atlantic Coast Conference and a runner-up position in the ACC Tournament in Charlotte, losing to
North Carolina by 5 points. The 2007–08 season marked Clemson's first appearance in the NCAA Tournament in ten years. However, Purnell was unable to win an NCAA tournament game (0–6) during his stints with ODU, Dayton and Clemson. On March 18, 2008, Clemson extended Purnell's contract through 2014 and raised his salary. On April 6, 2010, Purnell signed a seven-year deal with
DePaul University. At the conclusion of the
2014–2015 season, Purnell announced his resignation. ==Head coaching record==