An electrical box mounting bracket includes a mounting frame having a central opening. A removable support member is coupled to an inner edge of the central opening by a frangible tab. A removable extension member is coupled to the support member by a frangible tab that can be removed and attached to an end of the support member. A cable guide is coupled to an inner edge of the central opening by a first fold line and is bendable to a position for supporting a cable. The cable guide has a body or support plate with a plurality of adjacent cable openings with a retaining tab between two adjacent cable openings. The retaining tab is bendable from a first open position for inserting a cable into the cable openings and a second closed position to retain the cable in the cable openings.