Background: The quality of life (QOL) perceptions reported by adults with intellectual disability (ID) might not reflect their QOL as assessed by support staff or family members. The main purpose of this research was to examine the consistency and agreement between self-report and report-of-others ratings and to analyse if there are predictors for the differences between these respondents. Method: For each person with an ID who completed a self-report measure (n = 69), a report-of-others section was conducted by interviewing one key institutional worker and one family member. QOL was assessed by the Escala Pessoal de Resultados, the Portuguese version of the Personal Outcomes Scale. Results: The findings highlighted a sizeable agreement among people with ID, support staff, and family members, although significant differences were found between the respondents. Among other determinants, the health status of the people with ID was the factor that most significantly contributed to the disagreement among the participants. Conclusions: The results indicated that the QOL scores among respondents did not seem to be interchangeable, and different perspectives should be included in QOL assessments.