Magnesium is the second most common intracellular cation after potassium and plays pivotal role in the majority of metabolic process. Several studies have shown the prevalence of hypomagnesemia ranging from 2.5% to 12% in general population and even up to 60% in intensive care unit patients. Hypomagnesemia might be more prevalent in patients with cancer owing to a combination of several factors such as gastrointestinal loss, renal loss, poor intake, and use of certain chemotherapeutic drugs. It is imperative that we identify the exact cause of hypomagnesemia to aid and guide treatment. We report a case of a 63-year-old white woman with hypomagnesemia who was undergoing treatment for metastatic colon cancer. The chemotherapy regimen was with FOLFIRI (folinic acid, 5-fluorouracil, and irinotecan) and bevacizumab. This was followed by maintenance therapy with Xeloda (capecitabine). Her hypomagnesium was attributed to her chemotherapy. During our workup, the renal fractional excretion of magnesium was found to be low excluding the cause as renal wasting. This patient's hypomagnesemia could very well be explained by gastrointestinal losses (diarrhea) from short bowel after colectomy, her chemotherapeutic agents and metformin, as well as poor oral intake from medications, or malignancy itself.