During the COVID-19 pandemic, countries adopted mitigation strategies to reduce disruptions to cancer services. We reviewed their implementation across health system functions and their impact on cancer diagnosis and care during the pandemic. A systematic search was performed using terms related to cancer and COVID-19. Included studies reported on individuals with cancer or cancer care services, focusing on strategies/programs aimed to reduce delays and disruptions. Extracted data were grouped into four functions (governance, financing, service delivery, and resource generation) and sub-functions of the health system performance assessment framework. We included 30 studies from 16 countries involving 192,233 patients with cancer. Multiple mitigation approaches were implemented, predominantly affecting sub-functions of service delivery to control COVID-19 infection via the suspension of non-urgent cancer care, modified treatment guidelines, and increased telemedicine use in routine cancer care delivery. Resource generation was mainly ensured through adequate workforce supply. However, less emphasis on monitoring or assessing the effectiveness and financing of these strategies was observed. Seventeen studies suggested improved service uptake after mitigation implementation, yet the resulting impact on cancer diagnosis and care has not been established. This review emphasizes the importance of developing effective mitigation strategies across all health system (sub)functions to minimize cancer care service disruptions during crises. Deficiencies were observed in health service delivery (to ensure equity), governance (to monitor and evaluate the implementation of mitigation strategies), and financing. In the wake of future emergencies, implementation research studies that include pre-prepared protocols will be essential to assess mitigation impact across cancer care services.