Magnesium is involved in the regulation of blood pressure (BP). Abnormalities in serum magnesium are common in chronic kidney disease (CKD), yet its association with the development of hypertension and CKD progression in patients with CKD is unclear. We analyzed data from 3866 participants from the CRIC Study (Chronic Renal Insufficiency Cohort). Linear regression assessed the association of serum magnesium with baseline systolic BP (SBP) and diastolic BP (DBP). Logistic regression explored the association of serum magnesium with various definitions of hypertension. Cox proportional hazards models explored assessed the risk of incident hypertension and CKD progression. Mean serum magnesium was 2.0 mEq/L (±0.3 mEq/L). Higher magnesium was associated with lower SBP (−3.4 mm Hg [95% CI, −5.8 to −1.0 per 1 mEq/L]) and lower DBP (−2.9 mm Hg [95% CI, −4.3 to −1.5 per 1 mEq/L]). Higher magnesium was associated with a lower risk of American Heart Association–defined hypertension (SBP≥130 mm Hg or DBP≥80 mm Hg) at baseline (adjusted hazard ratio, 0.65 [95% CI, 0.49–0.86 per 1 mEq/L]), a lower risk of suboptimally controlled BP (SBP≥120 mm Hg or DBP≥80 mm Hg; adjusted odds ratio, 0.58 [95% CI, 0.43–0.78 per 1 mEq/L]). In time-to-event analyses, higher baseline serum magnesium was associated with a nominally lower risk of incident CRIC-defined hypertension (adjusted hazard ratio, 0.77 [95% CI, 0.46–1.31 per 1 mEq/L]). Higher magnesium was associated with a significantly lower risk of CKD progression (adjusted hazard ratio, 0.68 [95% CI, 0.54–0.86 per 1 mEq/L]). In patients with CKD, higher serum magnesium is associated with lower SBP and DBP, and with a lower risk of hypertension and CKD progression. In patients with CKD, whether magnesium supplementation could optimize BP control and prevent disease progression deserves further investigation.