Stress-assisted grain growth in nanocrystalline metals transpires collectively with a number of competing deformation mechanisms. In this study, molecular dynamics simulations of surface nanoindentation were performed to quantify the plastic strain distribution among competing mechanisms during stress-assisted grain growth in nanocrystalline Ni and Ni-P. In the finest grain size nanocrystalline metals, mechanical grain growth was attributed to grain boundary-mediated deformation mechanisms involving grain boundary migration and grain rotation. The manifestation of these mechanisms in the deformation tensor was quantified by isolating specific grain coalescence events. Using these continuum metrics, a reduction in deformation temperature, increase in nanocrystalline grain size, or addition of P segregated to the grain boundaries were found to quell mechanical grain growth by suppressing grain boundary-mediated plasticity. In the absence of grain boundary deformation mechanisms, plastic strain was accommodated by dislocation plasticity, thus signifying the role of mechanistic crossovers in stress-assisted grain growth.