Fabricating high-performance nanoparticles (NPs) is currently a focus of researchers due to their manipulative size-dependent unique properties required to develop next-generation advanced systems. To harness the unique properties of NPs, maintaining identical characteristics throughout the processing and application process system is crucial to producing uniform-sized, or monodisperse, NPs. In this direction, mono-dispersity can be achieved by exerting extreme control over the reaction conditions during the NP synthesis process. Microfluidic technology offers a unique approach to control fluid conditions at the microscale and is thus well-positioned as an alternative strategy to synthesize NPs in reactors demonstrating micrometric dimensions and advanced size-controlled nanomaterial production. These microfluidic reactors can be broadly classified as active or passive based on their dependence on external energy sources. Passive microfluidic reactors, despite their lack of reliance on external energy, are frequently constrained in terms of their mixing efficacy when compared to active systems. However, despite several fundamental and technological advantages, this area of research as well as its application to the biological sciences is not well-discussed. To fill this gap, this review for the first time discusses various strategies for synthesizing NPs using active microfluidic reactors including acoustic, pressure, temperature, and magnetic assisted microfluidic reactors. Various established ways for achieving size control on NP synthesis in microfluidic reactors representing the applicability of micro-reaction technology in developing novel nanomaterials suitable for potential biomedical applications are presented in this review along with a comprehensive discussion about the challenges and prospects.