The support unit is essentially a box-like structure having a bottom, a top, two opposing side walls, a front door, a locking device for the front door, and a rear wall opposing the front door. The front door is pivotally attached to the side walls and positioned to allow a tool box or other container to be removed from, or placed into the support unit when the door is open and to prevent removal of the container when the door is closed. The container support unit is designed to securely hold a toolbox, or other box-like container, such as a U.S. Army-issue ammunition can. The support unit can be mounted to the side wall of the cargo area of a pickup truck, under the top rail of the cargo area sidewall, in front or behind the wheel well and suspended above the floor of the cargo area. The support unit can also be mounted on other vehicles, such as a boat, or any other suitable structure, such as a garage wall.