Socio-economic status (SES) is linked to the development of cognitive abilities, particularly language and executive processes. It is unclear whether these represent a single or independent correlates. We studied 110 Ecuadorian youths aged 12–17 with measures of SES, language, executive function, and theory of mind (ToM), a.k.a. mentalizing. A subsample gave hair samples to estimate recent cortisol levels. Restricting analyses to reliable measures, SES was highly associated with language skill, and to a lesser extent with executive function and ToM performance. However, those latter associations were attenuated and non-significant when language ability was controlled for statistically. Systemic cortisol levels were not associated with SES, but were significantly and negatively correlated with ToM, independent of variation in language skills. We conclude that language development underlies most of the impact of SES on executive function and ToM ability of adolescents, but that stress-related cortisol may have an independent, direct effect on mentalizing.