Objective: To minimize risk and prevent harmful incidents during interhospital transport, the critical care transport unit service called Seoul Mobile Intensive Care Unit (SMICU) was organized and initiated its service within the city of Seoul. We sought to evaluate the effectiveness of critical care transport units on outcomes of critically ill patients undergoing interhospital transport in Seoul. Methods: A retrospective observational case–control study was designed to evaluate the effectiveness of critical care transport units on outcomes of critically ill patients undergoing interhospital transport. ED patients transported from other hospitals in Seoul during 2016 were identified in the National Emergency Department Information System (NEDIS) and according to use of the SMICU. One-to-one propensity matching was performed to balance covariates between groups. The association of SMICU transport on survival outcome was calculated in a multivariable logistic regression model. Results: Among 42,188 ED patients transported from other hospitals in 2016, 482 (1.1%) of patients were transported by SMICU. Patients transported by SMICU had a higher proportion of severe emergency disease and use of a mechanical ventilator. The adjusted odds ratio for 24-hour mortality after interhospital transport was 0.45 (95% CI: 0.26–0.81) in total cohort and was 0.34 (95% CI: 0.16–0.71) in a one-to-one propensity-matched cohort. Conclusions: Transport by specialized critical care transport unit for patients undergoing interhospital transport was associated with lower 24-hour mortality, demonstrating the benefits of the SMICU.