Background: Previous studies have evaluated the impact of oral magnesium on blood pressure; however, they used magnesium salts with low bioavailability, had methodological issues, and showed differing results. Objective: To evaluate the long-term blood pressure–lowering ability of oral magnesium L-lactate versus placebo in patients with implanted cardioverter defibrillators (ICDs). Methods: In this double-blind, 24-week trial, 50 patients with ICDs were randomized to receive magnesium L-lactate (6 tablets daily, supplying a total of 504 mg of elemental magnesium daily) or matching placebo for at least 12 weeks. Baseline intracellular and serum magnesium concentrations were determined. The primary efficacy endpoint was the mean sitting systolic blood pressure at 24 weeks. Results: In 50 patients who completed at least 12 weeks of follow-up, 86% of patients, regardless of randomization, had a baseline intracellular magnesium deficiency, but no patients had a serum magnesium deficiency. At 12 weeks, magnesium L-lactate significantly reduced systolic blood pressure compared with placebo (117.7 ± 11.8 vs 126.3 ± 16.7 mm Hg, respectively; p = 0.04). In the 45 patients who continued in the study through the 24-week time period, the systolic blood pressure reduction was maintained, but statistical significance was lost (118.1 ± 14.1 vs 125.5 ± 17.2 mm Hg, respectively; p = 0.13). Magnesium L-lactate did not impact diastolic blood pressure at either time period (p ≥ 0.75 for both). Patients with a documented history of hypertension at baseline showed similar qualitative results to the primary analysis. Conclusions: A large number of subjects with ICDs have an intracellular magnesium deficiency not captured through serum magnesium determination. The use of magnesium L-lactate in patients with an ICD results in significant improvement in systolic blood pressure at 12 weeks, which may be maintained through 24 weeks.