Chronic magnesium (Mg) deficiency has been associated with hypertension and obstructive sleep apnea in many species; brachycephalic dogs are at increased risk for these conditions. This study aimed to retrospectively compare the prevalence of hypomagnesemia (HM) in boxers and bulldogs; prospectively screen a group of young healthy bulldogs for HM, and; explore the use of parenteral magnesium tolerance testing (PMgTT) to diagnose whole-body Mg deficiency in dogs. Of 208 boxers and 128 bulldogs, 4.7% and 15% were found to be hypomagnesemic, respectively. Risk ratio of HM was 1.8 in bulldogs compared to boxers. All but 1 of 16 bulldogs had total Mg levels at or below the lowest reference value. PMgTT was performed in 3 of these dogs and results suggested moderate to severe whole body Mg deficiency. Based on current evidence, the authors suggest decreased gastrointestinal absorption of Mg in bulldogs likely plays a role in HM. Additionally, recurrent episodes of hypercapnia caused by sleep apnea may cause increases in urinary Mg excretion, further exacerbating hypercapnia. Mg deficiency appears common in bulldogs and likely contributes to disease processes including arterial hypertension and sleep apnea. Ionized-to-total Mg ratio and PMgTT may prove helpful in diagnosing chronic subclinical Mg deficiency.