This case report describes severe hypomagnesemia in a cat attributed to refeeding syndrome with an onset of clinical signs from the magnesium deficiency apparent on the twelfth day following initiation of feeding. The patient initially presented in a state of cachexia from apparent malnutrition after missing from the owners care for five months. The patient was initially discharged five days after the initiation of feeding with only a mild hypokalemia apparent and requiring supplementation and returned for outpatient management. The patient presented through the emergency department on the twelfth day following the onset of feeding with the clinical signs of acute lethargy, vomiting, generalized tremors and a seizure episode and had a severe total hypomagnesemia on diagnostic bloodwork. The patient's clinical signs resolved following emergency treatment with parenteral magnesium sulfate as a continuous rate infusion and was later managed with oral magnesium hydroxide for a prolonged period of time. Electrolyte abnormalities and associated clinical signs typically occur between two and five days after initiation of feeding and up to ten days after starting food intake in humans with anorexia nervosa. This case report highlights that hypomagnesemia, while not the most common electrolyte disturbance to occur with refeeding syndrome, can occur without other significant electrolyte changes and can cause clinical signs greater than ten days following refeeding to a starving patient. This magnesium deficiency required prolonged treatment, but the patient made a complete recovery.