INTRODUCTION: The COVID-19 pandemic has disrupted health systems around the world. The objectives of this study are to estimate the overall effect of the pandemic on essential health service use and outcomes in Mexico, describe observed and predicted trends in services over 24 months, and to estimate the number of visits lost through December 2020. METHODS: We used health information system data for January 2019 to December 2020 from the Mexican Institute of Social Security (IMSS), which provides health services for more than half of Mexico's population-65 million people. Our analysis includes nine indicators of service use and three outcome indicators for reproductive, maternal and child health and non-communicable disease services. We used an interrupted time series design and linear generalised estimating equation models to estimate the change in service use and outcomes from April to December 2020. Estimates were expressed using average marginal effects on the risk ratio scale. RESULTS: The study found that across nine health services, an estimated 8.74 million patient visits were lost in Mexico. This included a decline of over two thirds for breast and cervical cancer screenings (79% and 68%, respectively), over half for sick child visits and female contraceptive services, approximately one-third for childhood vaccinations, diabetes, hypertension and antenatal care consultations, and a decline of 10% for deliveries performed at IMSS. In terms of patient outcomes, the proportion of patients with diabetes and hypertension with controlled conditions declined by 22% and 17%, respectively. Caesarean section rate did not change. CONCLUSION: Significant disruptions in health services show that the pandemic has strained the resilience of the Mexican health system and calls for urgent efforts to resume essential services and plan for catching up on missed preventive care even as the COVID-19 crisis continues in Mexico.