Senior Investigating Officers investigate and prevent the most complex and serious criminal offences, including homicide, child homicide, terrorism, and serious organised crime. Yet, the wellbeing effects of being an SIO have not yet been delineated in the literature. The primary aim of this research was to understand the relationships between an SIOs working environment, and their wellbeing. To do this, a convergent-parallel mixed method research design was employed to quantify and understand the relationships between commonly experienced police stressors, and mental health. The research employed both a quantitative survey, and qualitative interviews. The research aimed to understand patterns in appraisal and coping processes through combining Reflexive Thematic Analysis (Braun & Clarke, 2006) and Lazarus and Folkman’s (1984) model of stress and coping. The findings demonstrate that organisational and operational police stress are significantly related to anxiety, depression, PTSD, and psychiatric distress in SIOs. The findings also demonstrate no overall differences between male and female SIOs on anxiety, depression, and psychiatric distress measures, although male SIOs reported higher PTSD scores. Further, item-level differences were noticed, with male SIOs scoring higher on ‘stigma’ and ‘social’ related stressors, whilst female SIOs reported greater fatigue and unequal sharing of work responsibilities. The research found that whilst organisational stress was significantly higher than operational stress, operational stress had a greater impact on psychiatric distress. The qualitative research comprised of 17 semi-structured interviews with SIOs across a variety of contexts, including: disaster investigations, terrorism, homicide, child homicide, and serious organised crime. The Reflexive Thematic Analysis generated six themes: ‘Doing too much with few resources’; ‘Demand as constraining coping’; ‘Being the safe pair of hands’; ‘The buck stops with you’; ‘Understanding’ as a buffer against unnecessary barriers’; and ‘Protecting the self through avoidance, detachment, and positive reappraisal’. This original contribution shows how SIOs contextually appraise stressors, through understanding ‘centrality’ and the meanings attached, and further demonstrates that SIOs can vary coping strategies according to time, place, and wider contexts. The research has further advanced knowledge of SIO coping by showing how different forms of coping are facilitative, by demonstrating how SIOs engage in avoidance and detachment-based coping in order to facilitate problem solving and positive reappraisal. Through a synthesis of research findings, this original contribution to the literature calls for a redressing of the balance in how SIOs manage and cope with the stress involved in their role, and the wellbeing impacts this has.