Mechanical twinning is a form of permanent deformation that occurs in various crystalline solids, especially in low symmetry crystals, such as hexagonal close-packed (HCP) metals. The presence and formation of twins have been shown to have drastic effects on the strength and ductility of materials. With recent increased interest in using HCP metals, such as magnesium, in structural, automotive, and armor applications due to their high strength to weight ratio, there is a need for comprehensive modeling of deformation twinning to understand the material’s response to various loading conditions. Past studies on mechanical twinning have taken either a microscopic or macroscopic approach to the phenomenon. Microscopic approaches have utilized density functional theory or molecular dynamic simulations to explore the underlying physics in twins at the sub-grain scale; while macroscopic approaches have used crystal plasticity to model twins as a “pseudo-slip”, ignoring length scale effects, to obtain bulk responses. However, twins form collectively across multiple grains with complex local morphology propagating into bulk behavior. These mesoscale aspects have been less studied and so is the focus of the current work. We propose a mesoscale model where twinning is treated using a phase-field approach and dislocation slip is considered using crystal plasticity. Lattice reorientation, length-scale effects, interactions between dislocations and twin boundaries, as well as twin and slip interactions with grain boundaries are all carefully accounted for within the model. We present the model, the implementation using a novel approach of accelerated computational micromechanics, and demonstrate it using simulations on polycrystalline materials. We summarize the insights gained from these studies and the implications on the macroscale behavior of HCP materials.