Understanding how magnetic fields alter phase transitions is a practical obstacle in magnetocaloric refrigeration, where optimizing the magnetocaloric effect requires precise knowledge of how a magnetic field shifts transition temperatures and latent heat. This problem matters because Ni-Mn-Sn Heusler alloys offer a rare-earth-free pathway to solid-state cooling near room temperature. To address this, we developed a hybrid molecular dynamics–Landau free energy framework with weak magnetostructural coupling (E = 0.08 eV/µB²) and applied it to Ni-Mn-Sn under fields from 0 to 20 T. The method captures both first-order structural (martensitic) and second-order magnetic transitions simultaneously. Key results show that at the optimal composition of 16 at.% Sn, a 5 T field increases the Curie temperature by 20 K (from 298 K to 318 K), suppresses thermal hysteresis by 40% (from 28 K to 16.8 K), yields a maximum entropy change of − 15 J/kg·K, and achieves a refrigerant capacity of 148 J/kg at 5 T — 24% higher than the widely studied 15 at.% Sn composition. The broader implication is that modest magnetic fields can be used to engineer optimal magnetocaloric performance in Ni-Mn-Sn through precise composition control, enabling compact, rare-earth-free magnetic heat pumps for near-room-temperature refrigeration.