A considerable amount of historic masonry structures (HMS) are composed of irregular stone masonry and populate historic centres worldwide. However, few studies have systematically investigated the masonry texture’s (or masonry unit arrangement) influence on structural behaviour. The main difficulty stems from the pattern acquisition and the definition of adequate parameters that correlate the texture with the structural behaviour. To this aim, this paper presents a stochastic 2D coursed-rectangular masonry pattern generator that incorporates geometric quality indexes (QI) to generate random patterns with consistent masonry quality and structural behaviour. The generator’s input parameters separately consider the available stone units and the “virtual mason” skill level to cover the range of possible pattern qualities. Finally, it is shown that a selected set of QI-s correlate well with the structural behaviour and that deviations from the “rules of art” reduce masonry structural capacity.