Conventional iron determination methods, such as colorimetric assays, often lack sensitivity at low iron levels and are susceptible to interference from complex compositions in serum and plasma. In this study, we developed a rapid, sensitive, and accurate method for iron determination in 1 min utilizing an inexpensive benchtop 0.5-T micromagnetic resonance relaxometry (μMRR) system with acidification treatment of samples (pH < 1). The method yielded a highly linear calibration curve (R2 > 0.999) between the transverse relaxation rate R2 and iron concentration from 0.5 to 1000 μM, with a limit of detection (LOD) of 0.25 μM and a minimal assay volume of 5 μL. The accuracy of this method was validated by inductively coupled plasma mass spectrometry (ICP-MS) across a diverse range of biological samples. This μMRR-based approach offers a rapid and convenient alternative for serum and plasma iron measurements, which can substantially reduce clinical diagnostic time and support real-time iron monitoring for patients.