Social skill and language are known to relate, not least in the example of those with specific language impairment (SLI). However, most of the research examining this trend has been conducted on young primary school age children and the nature of the relationships is unclear. Furthermore, little is known about which young people in general have social difficulties and whether language, social cognition, and social skills are directly associated at this age. In this study, a large cohort made up of young people with a history of SLI (N = 134) and a typically developing (TD) group (N = 124) of the same age were followed up in their final year of compulsory schooling (aged 16). Language, social cognition, social skills, and functional social outcomes (friendships and levels of social activity) were assessed using tasks and questionnaires. Modest associations were found between social cognition, language, and social behaviours, the strongest being between language and social cognition. Regression analyses showed that as a combined group, the adolescents' functional social outcomes were most associated with expressive language, social skill, and social cognitive ability. However, the patterns differed when the groups were analysed separately, with social cognition playing more of a role for those with SLI. These findings suggest that poor language may play a complex role in adolescents' social development.