Forrest joined the coaching staff shortly after Jones was named head coach at LSU in April.
"I appreciate the opportunity given to me by Coach Jones and LSU," Forrest said. "There are some personal matters that have taken a priority that I need to give immediate attention."
Jones has begun a search to replace Forrest on the Tiger coaching staff.
"I will move quickly to fill out the coaching staff so we can continue preparations for the coming season," Jones said. "I thank Shawn for his contributions and will continue to support him in his future pursuits."