BackgroundThe COVID-19 pandemic has had a substantial impact on forensic mental health service provision and implementation. This study aimed to provide an analysis of the impact of COVID-19 related restrictions on routine outcomes within a large forensic mental health service in London, UK.MethodWe conducted a longitudinal cohort study using data collected routinely prior to the COVID-19 pandemic (April 2018–March 2020) and then stages thereafter (March 2020–March 2021; analyzed as March–May 2020, June–September 2020, October–December 2020, January–March 2021). We used causal impact models (Bayesian structural time-series) to examine the effect of COVID-19 related changes on routine outcomes related to service provision and implementation.ResultsThere was an overall increase in long-term segregation (LTS) hours during the pandemic; 140%, (95% Cl 107, 171%) during Lockdown 1; 113%, (159% Cl 127, 192%) during post-Lockdown 1; 45% (95% Cl 23, 68%) during Lockdown 2 and, finally, 90% (95% Cl 63, 113%) during Lockdown 3. The most negative outcomes were evident during Lockdown 3. Incidents of violence were significantly more frequent during Lockdown 3 than would have been predicted based on pre-pandemic data, including physical assaults to service users (206%, 95% CI 57%, 346%), non-physical assaults to service users (206%, 95% CI 53%, 339%), and self-harm (71%, 95% CI 0.4%, 135%). Use of enforced medication also increased during Lockdown 3 (317%, 95% CI 175%, 456%).ConclusionThe pandemic and its related restrictions negatively affected some service outcomes. This resulted in increased incidents of violence and increased use of restrictive interventions, beyond what would have been expected had the pandemic not occurred.