The first continuous synthesis of magnetic Fe@Fe3O4 core@shell nanoparticles with a metallic core is presented herein with precise control over size, narrow size distribution and a high production rate of 2.6 g per hour. This approach opens the door to large-scale production for their deployment in a range of applications such as drug delivery, separation, MRI contrasting agents, magnetically separable catalysts, magnetic hyperthermia for cancer treatment, etc. A systematic study of key reaction parameters in continuous microreactors reveal the main mechanistic steps involved in the thermal decomposition of the iron pentacarbonyl precursor. The presence of surfactants enables not only the post-synthesis particle stabilisation but also facilitates the initial ligand exchange in the precursor and the in situ CO production. We demonstrate that such gas production leads to a combined Dean-Taylor flow regime in the helical microreactors. Optimisation of the flow rate and reactor length leads to a high level of mixing and residence time (> 12 s) resulting in narrow size distribution and high precursor conversion respectively.