The synthesis of nanoparticles by the continuous flow process is of great interest since it allows extensive control over reaction conditions with high precision and provides enhanced production capacity with excellent heat and mass transfer rates at high pressures and temperatures. In this study, iron oxide nanoparticles were synthesized via continuous flow process in combined micro- and milli-sized reactors in the presence of citric acid at variable functional ratios from 1:1 to 1:5. The results illustrated the possibility of continuous production of superparamagnetic magnetite and/or maghemite nanoparticles at the size range of 4.3 to 4.6 nm that exhibit remarkable colloidal stability in triethylene glycol (TEG) and saturation magnetizations up to 52 emu/g. Additionally, the nucleation and growth stages of nanoparticles were found to be unaffected by the presence of citric acid (CA) while an increase in the functional ratio was shown to affect the magnetic properties due to the presence of a non-magnetic layer around the particles. Furthermore, the viability of human lung adenocarcinoma (A549) cell lines was investigated with several concentrations of magnetic nanoparticles, and the biocompatibility of nanoparticles was illustrated at certain particle loadings after 48 h expressing the potential use in biomedical applications.