BACKGROUND: The COVID-19 pandemic has presented many countries with significant health system and economic challenges. The role of civil-military cooperation in a health crisis of the magnitude presented by COVID-19 remains virtually unexplored. This review aims to detect and identify typologies, if any, of associations between security or military systems and the national response measures during the COVID-19, as adopted by six European countries during the early phase of the outbreak (January to March 2020). METHODS: We designed a structured qualitative literature review (qualitative evidence synthesis), primarily targeting open-source grey literature using a customised Google web search. Our target countries were UK, France, Spain, Italy, Belgium and Sweden. We employed a 'best fit' framework synthesis approach in qualitative analysis of the result records. RESULTS: A total of 277 result records were included in our qualitative synthesis, with an overall search relevance yield of 46%. We identified 19 distinct descriptive categories of civil-military cooperation extending across seven analytical themes. Most prominent themes included how military support was incorporated in the national COVID-19 response, including support to national health systems, military repatriation and evacuation, and support to wider public systems. CONCLUSION: Findings of this review show the significance of military systems in supporting an expansive response during the COVID-19 pandemic, and our proposed methodological approach for capturing military health data in a reproducible manner and providing a comparative view on common types of interventions provided by civil-military cooperation to inform lessons from the use of military capacities during current COVID-19 outbreak.