Background: The mental health of young people is a major public health concern that has been exacerbated during the COVID-19 pandemic. There is growing interest in cultural assets such as museums to improve mental health. However, there is a dearth of experimental research regarding the impact of cultural assets on common mental disorders in young adults, and in online experiences, which may improve accessibility.Methods/design: The O-ACE POP (Online Active Community Engagement Proof of Principle) study is a randomised controlled trial of a co-produced online museum experience named Ways of Being (WoB) compared with the Ashmolean Website (Ash) on negative affect, positive affect and symptoms of anxiety and depression (K10) over a 3-day intervention phase followed by a 6-week follow up assessment.Findings: 463 people aged 16-24 were randomised: 231 to WoB and 232 to Ash and 349 (74%) completed to 6 weeks. Negative affect was lower over the intervention phase in those allocated to WoB than Ash (WoB-Ash n=448, NA -0.158, p=0·010). For males (179, 39%), those aged 18-24 (422, 91%), non-white ethnic groups (88, 19%), regular users of online culture (238, 51%), those not on antidepressants (363, 78%), and individuals without probable moderate-severe mental health problems (327, 71%) at baseline, negative affect was significantly lower in those allocated to WoB compared to the Ash. Mean K10 reduced over the trial period, from 27·1 (95% CI= 26·6-27·7) to 23·3 (95% CI= 22·5-24·1), as did negative affect whereas positive affect remained relatively stable. Trial recruitment was rapid and feedback positive with broad geographical, occupational and ethnic diversity.Interpretation: Cultural interventions may improve MH outcomes in a measurable way. Populations that are underserved, both clinically and in research such as non-white ethnic groups, non-students and males and were reached in this study. This demonstrates the importance of measuring psychometric outcomes and the potential for co-produced community cultural resources to support mental health.Trial Registration: Trial Registration: NCT04663594Funding: This study is sponsored by the University of Oxford COVID-19 Research Response Fundand supported by the NIHR Oxford Health Biomedical Research Centre, The WestminsterFoundation and the Huo Family FoundationDeclaration of Interest: AKP and MV were supported by a grant provided by the Huo Family Foundation. All other authors have nothing to declare. Ethical Approval: The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and institutional committees on human experimentation and with the Helsinki Declaration of 1975, as revised in 2008. All procedures involving human subjects were approved by the University of Oxford Central University Research Ethics Committee (CUREC), approval reference number R70187/RE007.