Background: The Covid-19 pandemic has adversely affected population mental health. Methods: Using electronic health records from 1714 UK general practices registered with the Clinical Practice Research Datalink we examined incidence and event rates of depression and anxiety disorders, self-harm, prescriptions for antidepressants and benzodiazepines and GP referrals to mental health services per 100,000 person-months, before, during and after the peak of the Covid-19 emergency. Analyses were stratified by gender, age group and practice-level Index of Multiple Deprivation quintile. Findings: In April 2020, primary care-recorded incident depression reduced by 43·6% (95% CI 39·0% to 47·9%), anxiety disorders by 48·2% (CI 44·6% to 51·5%) and antidepressant prescribing by 36·2% (CI 33·7% to 38·6%) compared to expected rates based on prior trends. Reductions in first diagnoses of depression and anxiety disorders were particularly stark for working-age adults and patients registered at practices in more deprived areas. Self-harm incidence was 38·5% (CI 35·7% to 41·3%) lower than expected in April 2020. Total self-harm contacts fell by 28·2% (CI 25·5% to 30·8%). Rates of both incident and any self-harm remained around thirty percent lower than expected up to June 2020. Interpretation: Our findings reveal a stark treatment gap that was greater for first diagnoses of depression and anxiety disorders in working age adults, for practice populations in deprived areas, and for self-harm. Consequences could include more patients subsequently presenting with greater acuity and severity of mental illness and rising rates of non-fatal self-harm and suicide. Funding: This work was funded by the National Institute for Health Research (NIHR) Greater Manchester Patient Safety Translational Research Centre, UK Research and Innovation/Medical Research Council COVID-19 Rapid Response Initiative funding and by a University of Manchester Presidential Fellowship (SS). CC-G received funding from the NIHR Applied Research Collaboration West Midlands. Declaration of Interests: All authors have completed the Unified Competing Interest form (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years, no other relationships or activities that could appear to have influenced the submitted work. Ethics Approval Statement: This study is based on data from the Clinical Practice Research Datalink obtained under license from the UK Medicines and Healthcare products Regulatory Agency. The study was approved by the Independent Scientific Advisory Committee for Clinical Practice Research Datalink research (protocol number 20_094R1).