A modular retaining wall system uses a plurality of different sized masonry blocks to form uniform sized modules for constructing a segmented retaining wall. Each module has the same overall dimensions of height, width and depth, while the masonry blocks used to define the module vary in size and shape. Walls or structure faces have vertical or vertically set back surfaces are possible, using interlocking pins, channels and pin holes. A pin having an adjustable length is provided to accommodate masonry blocks of varying height.