Abstract: A modular wall-mounted electrical circuit device assembly includes an electrical junction box, a modular circuit device, and a cover plate. The junction box has an open front, and is wall-mounted with exposed ends of building wiring disposed therein. The modular circuit device includes a base unit and an interchangeable device module and is installed in the junction box via the open front. The base unit has a junction box liner defining a main cavity having an open front and a closed rear, exterior electrical connection points connected to the building wiring, electrical contacts disposed in the rear, and a first electrical interconnect assembly connecting the electrical contacts to the connection points. The interchangeable device module has a forward-facing circuit device, rearward-facing electrical contacts, and a second electrical interconnect assembly. The interchangeable device module is removably installed in the main cavity, via its open front, with the respective contacts plugged together.