The abiotic transformation of an amorphous iron (Fe) hydroxide hydrate to more crystalline Fe(III) minerals by Fe(II) plays an essential role in global Fe cycling. In natural environments, Fe(III) minerals generally coexist with organic matter, which modulates their mineralization pathways and byproducts. Nevertheless, the effect of exogenous organic matter, such as biochar, on the transformation of Fe(III) minerals remains unclear. In this study, a series of ferrihydrite-biochar complexes (Fh-BCs) with various C/Fe molar ratios were synthesized to evaluate the abiotic Fe(II)-catalyzed mineralogical transformation of Fh-BCs under neutral anaerobic conditions. During the synthesis of Fh-BC, biochar formed a complex with Fh through adsorption onto Fh at a C/Fe ratio of 0.3, whereas Fh loaded onto biochar with a C/Fe ratio of 1.2 generated the Fh-BC-1.2 complex. Compared to pure Fh, the specific surface area and total pore volume decreased in all of the Fh-BCs. The secondary mineral formation during Fh transformation depended on the C/Fe ratios. Biochar inhibited the formation of magnetite (Mgt) but not lepidocrocite (Lep) in the treatments of Fh-BC + Fe(II). However, the inhibition level of Mgt formation was negatively correlated with the C/Fe ratio. Experiments analyzing the time-dependent concentrations of Fe(II) and labile Fe(III) (Fe(III)_labile) against the kinetics of phase transformation showed that the occupation of adsorption sites on the surface of Fh by biochar inhibited electron exchange between Fh and Fe(II), thereby preventing the hydrolysis–reprecipitation of Fh into the more stable mineral phase. High C/Fe ratios modestly enhanced the transformation of Fh, which was attributed to the Fh-loaded structure that facilitated Fe(II) sorption and promoted efficient electron transfer between Fe(II) and Fh. These results indicate that biochar-modified Fh is favorable to the phase transformation of stable crystalline Fe(III) minerals, possibly providing new insight into the geochemical behavior of Fe/C cycling in carbon-rich soil environments.