The Mongolian Plateau grassland constitute a vital ecological barrier in the inland Asia. Within these ecosystems, the mineral-associated soil fraction (<53 µm, MASF) is the dominant component, and its iron (Fe) oxides play a pivotal role in mediating carbon (C), nitrogen (N), phosphorus (P) cycling. Using sequential chemical extraction, we quantified seven Fe fractions (Feex, Fecarb, Feox1, Feox2, Femag, Feprs, FeU) within the MASF across the plateau. The relative abundance of these fractions followed the sequence: FeU>Feprs>Feox2>Feox1>Femag>Fecarb>Feex. The combined Feox1 and Feox2 fractions, representing highly reactive Fe (FeHR), constituted 6.82–55.77% of total iron (FeT), identifying them as the dominant FeHR components. Both Fe fraction abundance and inorganic phosphorus extracted by sequential extraction (IPSE) decreased significantly along the grassland gradient: meadow steppe > typical steppe > desert steppe. This parallel decline underscores the key regulatory role of Fe (hydr)oxides in governing P fractionation and bioavailability within the MASF. Multivariate statistical analyses revealed soil physicochemical properties as the primary drivers of FeHR variability, explaining 64.52% of the variance, followed by climatic factors (18.6%) and vegetation factors (11.7%). IPSE drivers exhibited a similar hierarchy, suggesting a coupled geochemical cycling mechanism between FeHR and IPSE. This study provides fundamental geochemical insights into Fe fractionation within the MASF, advancing analytical approaches for understanding elemental cycling and ecological processes in Mongolian Plateau grasslands.