Background: We sought to (1) update estimates of the prevalence of significant cognitive delay (SCD) among nationally representative samples of young children overall, and in upper-middle, lower-middle and low-income countries; (2) investigate whether variation in prevalence between countries was systematically associated with national wealth and other country characteristics; (3) investigate the stability of prevalence estimates over time; (4) examine the correlation between SCD and 2019 Global Burden of Disease estimates on the prevalence of the impairment of developmental intellectual disability under 5 years of age; and (5) investigate the extent to which risk of SCD within countries varies with child age and gender, maternal education and household wealth. Methods: Secondary analysis of data collected in 126 nationally representative Multiple Cluster Indicators Surveys (MICS) conducted under the supervision of UNICEF in 73 countries involving a total of 396 596 3- to 4-year-old children. Results: The overall prevalence of SCD was 9.7% (95% CI 8.6–10.9%). Between-country variation in prevalence was strongly related to national wealth, the Human Development Index, the Human Inequality-adjusted Development Index and the Multidimensional Poverty Index, but not income inequality. In the 46 countries in which more than one survey was available prevalence estimates were reasonably stable over time (r = 0.80, P < 0.001). There were strong independent associations between increased risk of cognitive delay and younger child age, lower levels of maternal education and lower levels of household wealth (but not male gender). There was only a weak association across countries between the estimated prevalence of SCD and Global Burden of Disease estimates of the under 5 prevalence of the impairment of developmental intellectual disability. Conclusions: UNICEF's MICS data are readily (and freely) available to researchers and provide a cost-effective opportunity for researchers who are concerned about better understanding the situation of young children growing up in the world's LMICs with a marked loss of developmental potential in areas of cognition and learning.