Diagonal cracking is the most commonly reported mode of failure for masonry walls subject to in-plane shear loads. Finite element studies were therefore carried out to seek better understanding of this failure mode by attempting to capture the diagonal shear cracks in unreinforced concrete masonry and partially grouted/reinforced concrete masonry. Some key parameters (such as boundary and loading conditions, axial stress, and wall aspect ratio) were examined in the research. Interaction between axial stress and aspect ratio has been noticed. Diagonal compression “strut” was identified from contour plots of principal compressive stress, and therefore two distinct “strut” widths were proposed to account for strength and stiffness of the wall separately. Last but not least, effect of relative panel-to-“frame” stiffness ratio (represented by effective panel-to-frame modulus ratio in this study) has been observed through modelling in partially grouted/reinforced walls. This is rather an important step toward understanding the failure patterns of partially grouted masonry since this factor has not received much attention in the literature.