This article demonstrates the design and fabrication of an electrochemical sensor based on a platinum electrode for ascorbic acid (AA) modified by an electromagnetic polyaniline–Keggin iron–clay (PPICS) composite. The polyaniline–polyhydroxy iron–clay composite was prepared by the polymerization of [Anilinium]+[PDPSA]− in the presence of Keggin iron intercalated clay and was characterized for its particle size, morphology, electrical conductivity, and saturation magnetization. The oxidation potential of AA was observed at +0.4 V on the bare electrode and shifted to a negative potential of −0.32 V on the modified electrode. Common possible interferences of the sample matrices were tested, and the results reveal that the PPICS‐modified electrode exhibited a high selectivity and sensitivity toward AA. This unique low‐cost and user‐friendly sensor was validated for the nanomolar detection of AA present in real samples. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40936.