Abstract: A mounting system and method allows standard billboard panels to be mounted onto the side of a commercial semi-trailer or the like. A lower retaining channel and an upper retaining channel form a track into which billboard panels are inserted and retained. Removable end channels prevent the panels from sliding out of the track. The panels are secured together by tongue and groove joints and fasteners which are inserted through apertures in adjoining panels. The panels are covered with either paid commercial advertisements similar to a conventional billboard, or with the trailer owner's own name, insignia, advertising, or the like.