Objective: This study evaluated maintenance of improved delivery of smoking cessation assistance in adult acute psychiatry inpatient units 3 years post statewide implementation of a system change intervention through analysis of a statewide administrative health dataset. Method: Rates of documenting smoking status and providing a brief smoking cessation intervention (the Smoking Cessation Clinical Pathway) in all eligible Queensland public adult acute psychiatry inpatient units ( N = 57) during the implementation phase (October 2015–September 2017) of a system change intervention were compared to the maintenance phase (October 2017–October 2020) using interrupted time series analysis. Results: Across implementation and maintenance phases, the percentage of discharges from psychiatry inpatient units that had a smoking status recorded remained high with the statewide average exceeding 90% (implementation phase 93.2%, 95% confidence interval = [92.4, 93.9]; and maintenance phase 94.6%, 95% confidence interval = [94.0, 95.2]). The percentage of discharges statewide with a completed Pathway stabilised during the maintenance phase (change in slope −3.7%, 95% confidence interval = [−5.2, −2.3]; change in level 0.4%, 95% confidence interval = [−7.0, 7.9]). Conclusion: An evidence-based smoking cessation intervention implemented with a system change intervention resulted in sustained improvement in addressing smoking in adult inpatient psychiatry units up to 3 years post implementation.