Research on happiness has been of interest in many parts of the world. Here we provide evidence from developing countries; this is the first analysis of happiness among a cohort of Thai distance learning adults residing throughout the country (n = 60,569 in 2009). To measure happiness, we tested use of the short format Thai Mental Health Indicators (TMHI), correlating each domain with two direct measures of happiness and life satisfaction. Several TMHI domains correlated strongly with happiness. We found the mental state and the social support domains moderately or strongly correlated with happiness by either measure (correlation coefficients 0.24–0.56). The other two TMHI domains (mental capacity and mental quality) were not correlated with happiness. Analysis of socio-demographic attributes and happiness revealed little effect of age and sex but marital status (divorced or widowed), low household income, and no paid work all had strong adverse effects. Our findings provide Thai benchmarks for measuring happiness and associated socio-demographic attributes. We also provide evidence that the TMHI can measure happiness in the Thai population. Furthermore, the results among Thai cohort members can be monitored over time and could be useful for comparison with other Southeast Asian countries.