A modular interlocking exterior wall system comprises one or more laminated panels formed of a paper product core sandwiched between two cementitious skins, two vertical posts having laterally extending flanges which interlock in vertical grooves formed in the sides of the panels, and a top rail having a downwardly opening top groove which receives and reinforces the horizontal top edge of the top panel, wherein bottom portions of the posts are anchored below ground. An optional bottom rail has an upwardly opening groove for receiving and supporting the bottom edge of the bottom panel, and laterally extending flanges of an optional mid rail are received in top and bottom grooves, respectively, of bottom and top panels to interlock and reinforce the panels.