Background: Re-establishing societal norms in the wake of the COVID-19 pandemic will be important for restoring public mental health and psychosocial wellbeing as well as economic recovery. We investigated the impact on post-pandemic adjustment of a history of mental disorder, with particular reference to obsessive-compulsive (OC) symptoms or traits. Methods: The study was pre-registered (Open Science Framework; https://osf.io/gs8j2/). Adult members of the public (n=514) were surveyed between July and November 2020, to identify the extent to which they reported difficulties re-adjusting as lockdown conditions eased. All were assessed using validated scales to determine which demographic and mental health-related factors impacted adjustment. An exploratory analysis of a subgroup on an objective online test of cognitive inflexibility was also performed. Results: Adjustment was related to a history of mental disorder and the presence of OC symptoms and traits, all acting indirectly and statistically-mediated via depression, anxiety and stress; and in the case of OC symptoms, also via COVID-related anxiety (all p< .001). One hundred and twenty-eight (25%) participants reported significant adjustment difficulties and were compared with those self-identifying as “good adjusters” (n=231). This comparison revealed over-representation of those with a history or family history of mental disorder in the poor adjustment category (all p<.05). ‘Poor-adjusters’ additionally reported higher COVID-related anxiety, depression, anxiety and stress and OC symptoms and traits (all p< .05). Furthermore, history of mental disorder directly statistically mediated adjustment status (p< .01), whereas OC symptoms (not OC traits) acted indirectly via COVID-related anxiety (p< .001). Poor-adjusters also showed evidence of greater cognitive inflexibility on the intra-extra-dimensional set-shift task. Conclusion: Individuals with a history of mental disorder, OC symptoms and OC traits experienced greater difficulties adjusting after lockdown-release, largely statistically mediated by increased depression, anxiety, including COVID-related anxiety, and stress. The implications for clinical and public health policies and interventions are discussed.Funding: None to declare. Declaration of Interest: Prof. Naomi A. Fineberg declares that in the past 3 years she has held research or networking grants from the ECNP, UK NIHR, EU H2020, MRC, University of Hertfordshire; she has accepted travel and/or hospitality expenses from the BAP, ECNP, RCPsych, CINP, International Forum of Mood and Anxiety Disorders, World Psychiatric Association, Indian Association for Biological Psychiatry, Sun; she has received payment from Taylor and Francis and Elsevier for editorial duties. In the past 3 years, she has accepted a paid speaking engagement in a webinar sponsored by Abbott. Previously, she has accepted paid speaking engagements in various industry supported symposia and has recruited patients for various industry-sponsored studies in the field of OCD treatment. She leads an NHS treatment service for OCD. She holds Board membership for various registered charities linked to OCD. She gives expert advice on psychopharmacology to the UK MHRA. Prof. Samuel R. Chamberlain declares that his role in this research was funded by a Wellcome Trust Clinical Fellowship (110049/Z/15/Z & 110049/Z/15/A). Prof. Chamberlain previously consulted for Promentis (past 3 years) and receives a stipend from Elsevier for editorial work. Dr. Luca Pellegrini, Dr. David Wellsted, Natalie Hall, Prof. Ornella Corazza, Valentina Giorgetti, Dorotea Cicconcelli, Elena Theofanous, Nick Sireau, David Adam, Prof. Keith R. Laws report no financial relationships with commercial interests.Ethical Approval: The protocol and study objectives were pre-registered on 15th July 2020 (Open Science Framework; doi:10.17605/OSF.IO/GS8J2). Ethics approval was granted from the University of Hertfordshire (Ethics number: aLMS/SF/UH/04219).