Background: People with mental health conditions have been identified as particularly vulnerable to poor mental health during the coronavirus disease 2019 (COVID-19) pandemic. However, why this population have faced these adverse effects, how they have experienced them and how they have coped remains under-explored. // Aims: To explore how the COVID-19 pandemic affected the mental health of people with existing mental health conditions, and to identify coping strategies for positive mental health. // Methods: Semi-structured qualitative interviews with 22 people with mental health conditions. Participants were purposively recruited via social media, study newsletters and third sector mental health organisations. Data were analysed using reflexive thematic analysis. // Results: Participants were aged 23–70 (mean age 43), predominantly female (59.1%) and of white ethnicity (68.2%). Fifty percent were unable to work due to illness and the most frequently reported mental health condition was depression. Five pandemic-related factors contributed to deteriorating mental health: (i) feeling safe but isolated at home; (ii) disruption to mental health services; (iii) cancelled plans and changed routines; (iv) uncertainty and lack of control; (v) rolling media coverage. Five coping strategies were identified for maintaining mental health: (i) previous experience of adversity; (ii) social comparison and accountability; (iii) engaging in hobbies and activities; (iv) staying connected with others; (v) perceived social support. // Conclusions: Challenges were identified as a direct result of the pandemic and people with severe mental illnesses were particularly negatively affected. However, some found this period a time of respite, drew upon reserves of resilience and adapted their coping strategies to maintain positive well-being.