Understanding rock mechanical behavior, especially the onset and nature of yield (i.e. permanent deformation), is critical to ensure safe and sustainable operations in many geological engineering applications, including ‘low-carbon’ subsurface technologies to aid decarbonization. In porous sandstones, yield corresponds to onset of either permanent dilatation or compaction accompanied by microcracking and is defined by a ‘capped’ yield surface in differential stress vs effective mean stress (Q/P) space. Previous studies showed that compactive yield in porous sandstones corresponds to the onset of grain fragmentation and pore collapse, manifested by the deviation of the porosity reduction curve from the hydrostat. However, determining the onset of yield can be ambiguous. Here, we investigate the micromechanical basis for the compactive yield behavior of porous (25%) Hollington sandstone, which is comparable to common aquifers, hydrocarbon and geothermal reservoir rocks and other sandstone formations with potentials for underground storage of fluids (e.g. carbon dioxide and hydrogen). A suite of conventional triaxial compression experiments with concomitant pore volumometry were performed to characterize the mechanical behavior of the rock. Effective mean stress vs porosity reduction data show a slow but near-linear initial deviation from the hydrostat followed by a sharp increase in the rate of compaction for most samples that underwent inelastic porosity reduction below 243 MPa effective mean stress (P*). Synthesis of mechanical with microstructural data suggests that the initial deviation from the hydrostat is related to the collapse of local large pores while the sharp increase in the rate of compaction corresponds to the onset of grain fragmentation leading to widespread pore collapse. Plotting yield stresses in Q/P space reveal a near linear cap of negative slope, albeit with considerable scatter, for stresses related to the collapse of local large pores, evolving into an elliptical cap for stresses approximately corresponding to the onset of widespread grain fragmentation. The results highlight the enhancing influence of non-hydrostatic stresses acting around large pores on the compactive yield in rocks with porosity heterogeneities.