Impulsive behaviours affect patients with major depressive disorder (MDD) and bipolar disorder (BP), increasing suicide risk and mood instability. Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, has been reported to promote fast-acting antidepressive and antisuicidal effects. Magnesium administered contemporarily with low-dose NMDA antagonists has been shown to cause a reduction in anxiety-related and depressive-like behaviours. This observational study investigated the possible association between magnesium levels and impulsivity, measured using the Barratt Impulsiveness Scale (BIS-11), during ketamine treatment. Forty-nine inpatients with treatment-resistant mood disorders were involved in the study. The study therapeutic intervention was based on the administration of eight ketamine intravenous infusions over four weeks. The BIS-11 and magnesium levels were assessed for every subject before the first, third, fifth and seventh ketamine infusion and one week after the last infusion. The concentration of magnesium ions during the ketamine treatment was not associated with BIS-11 changes. The study does not provide evidence for a relationship between magnesium concentration and impulsivity, measured using the BIS-11, during ketamine treatment.