Background: This study was conducted to determine the association between fluid balance metrics and mortality and other postoperative outcomes after neonatal cardiac operation in a contemporary multicenter cohort. Methods: This was an observational cohort study across 22 hospitals in neonates (≤30 days) undergoing cardiac operation. We explored overall percentage fluid overload, postoperative day 1 percentage fluid overload, peak percentage fluid overload, and time to first negative daily fluid balance. The primary outcome was in-hospital mortality. Secondary outcomes included postoperative duration of mechanical ventilation and intensive care unit (ICU) and hospital length of stay. Multivariable logistic or negative binomial regression was used to determine independent associations between fluid overload variables and each outcome. Results: The cohort included 2223 patients. In-hospital mortality was 3.9% (n = 87). Overall median peak percentage fluid overload was 4.9% (interquartile range, 0.4%-10.5%). Peak percentage fluid overload and postoperative day 1 percentage fluid overload were not associated with primary or secondary outcomes. Hospital resource utilization increased on each successive day of not achieving a first negative daily fluid balance and was characterized by longer duration of mechanical ventilation (incidence rate ratio, 1.11; 95% CI, 1.08-1.14), ICU length of stay (incidence rate ratio, 1.08; 95% CI, 1.03-1.12), and hospital length of stay (incidence rate ratio, 1.09; 95% CI, 1.05-1.13). Conclusions: Time to first negative daily fluid balance, but not percentage fluid overload, is associated with improved postoperative outcomes in neonates after cardiac operation. Specific treatments to achieve an early negative fluid balance may decrease postoperative care durations.