An adjustable mounting system for wall-mounted shelf units is provided. The mounting system of the shelf unit comprises at least one horizontal support, situated preferably in proximity to the wall surface near the upper section of the shelf unit. The horizontal support is attached to, and slides laterally in clamping brackets that have been mounted to a wall. One part of the brackets are designed to be fastened to wall vertical support columns or studs, and incorporate a second portion that provides a clamping mechanism that attaches to the horizontal. The horizontal support is preferably fastened at each end to the shelf unit. One or more vertical columns are located between the shelves of the shelf unit and are located proximal, and optionally anterior to the horizontal support. A clamping element, which is preferably a plate with holes drilled into it, is placed proximate to the vertical support columns. The vertical support column and the horizontal support are clamped together using the clamping element and preferably with U-bolts. The nuts on the U-bolt are adjustable for clamping pressure. The shelf unit is particularly well suited for holding electronic equipment.