Development of new tools to improve the efficiency of iron minerals in promoting Fenton oxidation for environmental remediation is a highly promising field. Here, we examine for the first time the role of ascorbic acid (AA) in improving the magnetite (Fe3O4)-mediated Fenton oxidation to remove pentachlorophenol (PCP) in a historically contaminated soil. Experiments were performed in batch and flow-through conditions. In batch slurry experiments, the combination of Fe3O4/AA/H2O2 removed up to 95% of PCP as compared to the 43% removal by Fe3O4/H2O2. Dissolved Fe(II) measurements and Mössbauer spectroscopy highlight the role of AA in increasing the Fe(II) generation. Therefore, its presence enabled the Fe3O4 to maintain its structural Fe(II) content even after the oxidation reaction. Despite kinetic limitations in water-saturated columns, use of Fe3O4/AA/H2O2 removed about 70% of PCP contrary to the 20% PCP removal with Fe3O4/H2O2. This oxidation performance was affected by an injection flow rate or column residence time of AA and H2O2 in columns. Thus, the presence of AA significantly improved the ability of magnetite in promoting the Fenton reaction. Owing to the crucial role of AA in the Fe(II)/Fe(III) redox cycling, a mixed-valent character of magnetite makes it a potential catalyst for Fenton oxidation of organic pollutants.