Mutations in the <i>KCNA1</i> gene encoding the voltage-gated potassium (K⁺) channel Kv1.1 have been linked to rare neurological syndromes, episodic ataxia type 1 (EA1) and myokymia. In 2009, a <i>KCNA1</i> mutation was identified in a large family with autosomal dominant hypomagnesemia. Despite efforts in establishing a genotype-phenotype correlation for the wide variety of symptoms in EA1, little is known on the serum magnesium (Mg²⁺) levels in these patients. In the present study, we describe a new de novo <i>KCNA1</i> mutation in a Polish patient with tetany and hypomagnesemia. Electrophysiological and biochemical analyses were performed to determine the pathogenicity of the mutation. A female patient presented with low serum Mg²⁺ levels, renal Mg²⁺ wasting, muscle cramps, and tetanic episodes. Whole exome sequencing identified a p.Leu328Val mutation in <i>KCNA1</i> encoding the Kv1.1 K⁺ channel. Electrophysiological examinations demonstrated that the p.Leu328Val mutation caused a dominant-negative loss of function of the encoded Kv1.1 channel. Cell surface biotinylation showed normal plasma membrane expression. Taken together, this is the second report linking <i>KCNA1</i> with hypomagnesemia, thereby emphasizing the need for further evaluation of the clinical phenotypes observed in patients carrying <i>KCNA1</i> mutations.