Single-item measures of general well-being are increasingly being analysed cross-culturally but without clear evidence of comparability level attainment. The primary objective of this study is to examine the cross-country measurement invariance of the two most common single-item measures—life satisfaction and happiness—across a large number of countries. For this purpose, 45 data sources from large-scale sample surveys conducted between 1976 and 2018 were used. This study presented a novel technique for examining the measurement invariance of individual items and used Bayesian approximation to evaluate the extent of the non-invariance of certain items across nations. The findings revealed that the happiness item’s factor loadings and intercepts deviated less, indicating comparability across more countries than the life satisfaction item. It is possible that the construct of happiness is more universally applicable across cultures than that of life satisfaction. However, the item parameters of the survey items varied among several countries in each round of the program, indicating that the observed score means could only be compared between a few participating countries.