Cetuximab (Cmab) is known to cause electrolyte abnormalities, including hypomagnesemia, hypokalemia, and hypocalcemia. However, little is known about differences in these electrolyte levels between hypomagnesemia and non-hypomagnesemia group in patients receiving Cmab. Therefore, the aim of this study was to clarify the relationship between these serum electrolyte levels in patients undergoing Cmab therapy. A retrospective study was conducted to investigate patients for advanced or recurrent colorectal cancer and head and neck cancer, treated with a regimen composed of Cmab from 2012 to 2020 at the Kindai University Nara Hospital. A total of 113 patients were identified from the medical records, and 24 patients who met the inclusion criteria were enrolled in this study. In the non-hypomagnesemia group, significant positively correlations were observed between calcium and potassium (p = 0.018), between potassium and magnesium (p = 0.008), and between magnesium and calcium (p = 0.038). Simultaneously, in the hypomagnesemia group, no statistically significant correlations were recorded between these electrolytes. The incidences of hypokalemia, hypomagnesemia, and hypocalcemia were 25.0% (6/24), 29.2% (7/24), and 25.0% (6/24), respectively. Additionally, the onset of hypokalemia was significantly associated with the onset of hypocalcemia (p = 0.009). These data suggest that it is important to monitor these electrolyte levels, especially in patients who received Cmab with combination therapy.