Microplastics (MPs) acted as environmental emerging pollutants have been widely distributed in natural waters. Due to their small particle size and large specific surface area, MPs can be adsorbed or accumulated in aquatic organisms, resulting in potential toxicity to mammals or humans through dietary routes. In this review, we discussed the distribution of MPs in natural waters, aquatic organisms, and humans. Pretreatment methods for the determination of MPs in biological samples were summarized, which are closely related to tissue composition, especially for human samples. Further elucidation of the potential toxicity and underlined mechanisms of MPs in mammals was carried out to parallel it in humans. This review provides a comprehensive understanding of the ecological and health risks of MPs originating from natural waters, as well as perspectives for more specific studies on the toxicity of MPs to mammals or humans.