The generation of runaway electrons (REs) during the current quench of a disruption is often accompanied by strong magnetohydrodynamic (MHD) activity. MHD instabilities and REs can interact in several ways, making it important to self-consistently model RE interactions with MHD activity in disrupting plasmas, for accurate predictions of RE generation, and the design of mitigation strategies, such as massive gas injection (MGI) and passively driven mitigation coils. Using M3D-C1, which is an extended MHD code that uses a RE fluid model, we investigate the effect of 3-D nonlinear MHD activity on RE evolution and plateau formation during disruptions on SPARC – a compact, high-field, high-current tokamak designed to achieve a fusion gain Q > 2 in deuterium–tritium plasmas. In the unmitigated disruption, where a dominant m/n = 1/1 mode produces cyclic sawtooth-like activity, we observe an enhancement of RE generation by MHD activity, without any significant RE losses. On the other hand, in a disruption mitigated via neon MGI, the resulting MHD activity generates completely stochastic field lines, producing a complete loss of the RE population. Re-healing of flux surfaces in the core thereafter enables confinement and growth of REs via avalanching, and a plateau RE current, peaked at the core, is obtained. Our results demonstrate that the magnitude, longevity, and the spatial localization of MHD modes play an important role in deconfining REs in a disruption plasma. *This work is supported by Commonwealth Fusion Systems.