Therrell began his coaching career at the high school level, serving for one year at
Campbell County High School (1979) and three seasons at
Rhea County High School (1980–1982) in Tennessee. He made his first entry into major college football as a
graduate assistant coach at the
University of Tennessee. In 1985, he became an
assistant football coach for
Middle Tennessee State University, coaching the Blue Raiders
defensive ends until 1987. He then served as the
defensive coordinator for Division-II member
Lock Haven University for three years (1987–1989), before becoming the school's
head coach in 1990. In 1996, Therrell joined
Todd Berry's coaching staff at
Illinois State University as the defensive coordinator. He helped to lead the
Redbirds to the
1999 NCAA Division I-AA semifinals. Following the 1999 campaign, Therrell followed Berry to the
United States Military Academy, where he served as the defensive coordinator for four seasons (2000–2003). He then was the defensive coordinator for
Murray State University from 2004 to 2005. The
Murray State Racers football team defense was ranked fourth. Subsequently, Therrell had a three-year stint at
UNLV from 2007 to 2009, serving as the Rebels defensive coordinator for the last two seasons. He also coached the safeties in 2009, after spending his first two seasons mentoring the linebackers. After coaching three seasons in the professional ranks with the
Las Vegas Locomotives of the
United Football League (UFL), Therrell returned to Murray State as defensive coordinator in 2013. Therrell served as head coach at his alma mater,
Bledsoe County High School, during the fall of 2017, leading the Warriors to a 4–7 record. He resigned in July 2018. ==Head coaching record==