Microplastics (MPs) and heavy metals are emerging environmental contaminants of increasing concern due to their potential synergistic impacts on subsurface ecosystems. This study examined the combined influence of MPs—polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET)—on the adsorption–desorption dynamics and vertical migration of hexavalent chromium [Cr(VI)] in aquifer systems underlying municipal solid waste (MSW) landfills. Batch experiments demonstrated that red-bed soils exhibited a high adsorption capacity for Cr(VI) (101.83 mg·kg⁻¹), primarily governed by chemisorption and partial reduction to Cr(III). However, the presence of MPs disrupted these processes, reducing adsorption efficiency by up to 16.46% while enhancing desorption by 20.92%. Column transport experiments further revealed that MPs significantly accelerated Cr(VI) migration, particularly at higher concentrations (5–10%) and with larger particle sizes. The maximum migration rate reached 0.079 cm·h⁻¹, representing a 51.92% increase compared with the control, while the penetration time was reduced to 25 days. Redox analysis indicated that MPs inhibited the transformation of Cr(VI) to Cr(III), thereby weakening the natural attenuation capacity of the soil. Collectively, these results demonstrate that MPs intensify the mobility and persistence of toxic heavy metals in subsurface environments of landfill systems. The findings underscore the potential environmental risks associated with co-contamination, highlighting the need for integrated risk assessments and management strategies that account for interactions between MPs and heavy metals in landfill-affected aquifers.