Microplastics (MPs) have infiltrated soil–water–sediment systems pervasively through terrestrial, freshwater and marine compartments, leading to lingering pollution while secondary microplastics account for the most environmental loads including wastewater treatment plants, agricultural plastics, landfill leachate and tire wear as major diffuse sources. The transport of MPs proceeds via coupled hydrodynamic–particle interactions, heteroaggregation with suspended sediments and biofouling processes that contribute to sedimentary burial with resuspension dynamics serving as a long-term reservoir storing the legacy burdens and sustain trophic exposure long after primary emissions. Ecotoxicological evidence replaces MPs as inert vectors with bioactive disruptors which are activated via hierarchical stress pathways that induce oxidative stress, immunomodulation and metabolic impairments with the polymer-specific toxicity and temperature-dependent effects. Direct dietary exposure pathways are established by both trophic transfer and pathways for crop uptake through apoplastic and symplastic routes with having human health implications detected in atherosclerotic plaques, placental tissue and multiple organ compartments, showing mechanistic evidence of mitochondrial dysfunction and inflammatory signalling. The removal efficiencies in current mitigation technologies are high, but current contamination cannot be reversed, hence the need for a shift from end-of-pipe remediation to source-directed regulation. Critically collating the range from the environmental sources to the nutritional effects, the review uncovers salient gaps in knowledge, notably around nanoplastic identification, crop transfer coefficients and sediment-inclusive risk modelling and establishes a cohesive platform to inform future knowledge and policy direction within the One Health space.