Background: Few studies have evaluated the explanatory factors of poor performance and the effects of context in adults with intellectual disabilities. The aim of this study is to assess the role of a familiar experimenter on their cognitive performance, well-being, metacognitive experiences, and social behaviors. Method: Participants with moderate to severe intellectual disability were recruited into two groups, one with a familiar and one with an unfamiliar experimenter. They carried out a categorization task. Before and after they reported their metacognitive experiences and level of well-being. The experimenter observed their pro-social behavior. Results: Performance and some social behaviors were better when the participant knew the experimenter. However, he did not affect the level of well-being. The participants' metacognitive experiences were poor, whether or not they knew the experimenter. Conclusions: The familiarity of the experimenter plays a determining role, both on the participants' performance, and on their compliance with certain pro-social behavior.