Mechanical twinning is a form of inelastic deformation in magnesium and other hexagonal close-packed (hcp) metals, which has a drastic effect on material behavior. Magnesium’s high strength to weight ratio has led to its interest in structural, automotive, and armor applications, requiring a comprehensive understanding of twinning’s effect on material response. Past studies have taken either a microscopic approach, through molecular dynamics, or a macroscopic approach, through simplified pseudo-slip. However, twins interact across the mesoscale, forming collectively across grains with complex local morphology propagating into bulk behavior. To this end, we propose a variational model where twinning is treated using a phase-field approach, while slip is considered using crystal plasticity. Lattice reorientation, twinning length-scale, and twin-slip interactions arise naturally through energy minimization. We present GPU accelerated simulations on polycrystalline solids and summarize the insights gained from these studies and the implications on the macroscale behavior of hcp materials.