The Pediatric Cardiac Critical Care Consortium (PC4) cardiac arrest prevention (CAP) quality improvement (QI) project facilitated a decreased in-hospital cardiac arrest (IHCA) incidence rate across multiple hospitals. The sustainability of this outcome has not been determined. To examine the IHCA incidence rate at participating hospitals after the QI project ended and discern which factors best aligned with sustained improvement. This observational cohort study compared IHCA data from the CAP era (July 1, 2018, to December 31, 2019) with data from the 2-year follow-up era (March 1, 2020, to February 28, 2022). Data were obtained from pediatric cardiac intensive care units (CICUs) from 17 PC4 CAP-participating hospitals. The CAP practice bundle was designed to facilitate local practice integration, with the intention to implement, adapt, and continue CAP processes beyond the CAP era. A web-based survey was administered 2 years after the end of the project to estimate CAP-specific QI work. Risk-adjusted IHCA incidence rates across all admissions were compared between study eras. The survey generated a novel hospital-specific QI sustainability score, which is generally reflective of the sum of local CAP work performed. There were no clinically important differences in demographic and admission characteristics between the 13 082 CAP era admissions and 16 284 follow-up admissions (total mean [SD] age, 5.1 [8.4] years; 56.1% male). Risk-adjusted IHCA incidences were not different between the CAP vs follow-up eras (2.8% vs 2.8%; odds ratio, 1.03; 95% CI, 0.89-1.19), suggesting sustained prevention improvement. There was also no difference between eras in risk-adjusted IHCA incidence within medical, surgical, or high-risk subgroups. A lower hospital QI sustainability score was correlated with higher odds for IHCA in the follow-up vs CAP era (correlation coefficient, -0.58; P = .02). Five hospitals had increases of 1% or greater in risk-adjusted IHCA rates in the follow-up era; these hospitals had significantly lower QI sustainability scores and were less likely to have adopted sustainability elements during the CAP era or report persistent engagement for CAP-related QI processes during follow-up. In this cohort study of all CICU admissions across 17 hospitals, IHCA prevention was feasible and sustainable; the established reduction in risk-adjusted IHCA rate was maintained for at least 2 years after the end of the CAP project. Both implementation strategies and continued engagement in CAP processes during the follow-up era were associated with sustained improvement.