Latin American and Caribbean countries are dealing with the combined challenges of pandemic-induced socicoeconomic stress and increasing public debt, potentially leading to reductions in welfare and health-care services, including primary care. We aimed to evaluate the impact of primary health-care coverage on child mortality in Latin America over the past two decades and to forecast the potential effects of primary health-care mitigation during the current economic crisis. This multicountry study integrated retrospective impact evaluations in Brazil, Colombia, Ecuador, and Mexico from 2000 to 2019 with forecasting models covering up to 2030. We estimated the impact of coverage of primary health care on mortality rates in children younger than 5 years (hereafter referred to as under-5 mortality) across different age groups and causes of death, adjusting for all relevant demographic, socioeconomic, and health-care factors, with fixed-effects multivariable negative binomial models in 5647 municipalities with an adequate quality of vital statistics. We also performed several sensitivity and triangulation analyses. We integrated previous longitudinal datasets with validated dynamic microsimulation models and projected trends in under-5 mortality rates under alternative policy response scenarios until 2030. High primary health-care coverage was associated with substantial reductions in post-neonatal mortality rates (rate ratio [RR] 0·72, 95% CI 0·71-0·74), toddler (ie, aged between 1 year and <5 years) mortality rates (0·75, 0·73-0·76), and under-5 mortality rates (0·81, 0·80-0·82), preventing 305 890 (95% CI 251 826-360 517) deaths of children younger than 5 years over the period 2000-19. High primary health-care coverage was also associated with lower under-5 mortality rates from nutritional deficiencies (RR 0·55, 95% CI 0·52-0·58), anaemia (0·64, 0·57-0·72), vaccine-preventable and vaccine-sensitive conditions (0·70, 0·68-0·72), and infectious gastroenteritis (0·78, 0·73-0·84). Considering a scenario of moderate economic crisis, a mitigation response strategy implemented in the period 2020-30 that increases primary health-care coverage could reduce the under-5 mortality rate by up to 23% (RR 0·77, 95% CI 0·72-0·84) when compared with a fiscal austerity response, and this strategy would avoid 142 285 (95% CI 120 217-164 378) child deaths by 2030 in Brazil, Colombia, Ecuador, and Mexico. The improvement in primary health-care coverage in Brazil, Colombia, Ecuador, and Mexico over the past two decades has substantially contributed to improving child survival. Expansion of primary health-care coverage should be considered an effective strategy to mitigate the health effects of the current economic crisis and to achieve Sustainable Development Goals related to child health. UK Medical Research Council. For the Spanish and Portuguese translations of the abstract see Supplementary Materials section.