A protective case for housing a mobile device is provided. The protective case comprises a top shell that seals against a top surface of a device and a bottom shell that cradles the back of the device. The top shell provides a downward extension along its perimeter that fits within a pocket formed around at least a portion of the perimeter of the bottom shell. At least the top shell forms an outward extending protrusion that is gripped by one or more backing elements that slide over the exterior of the bottom shell and extend up around the protrusion to grip the top shell and secure it to the bottom shell. Where upper and lower backing elements are used that meet at a parting line along the outer (exterior) surface of the bottom shell, a locking mechanism may be used to pull the backing elements together and hold them in place.