Background This study explored the relationship between intelligence quotient (IQ) and aspects of everyday functioning/participation in individuals (age 16–40) who have a mild/borderline intellectual disability (IQ 55–85). Method Correlations were examined between IQ and (i) self‐rated (n = 72) ability, participation as performance (how often an activity is performed), important participation restriction (not/seldom performing an activity perceived as important) and general well‐being and (ii) proxy‐rated (n = 41) ability and participation as performance. Results No significant correlations between IQ and any of the explored measures were found. However, the effect sizes of the correlations between IQ and ability were considered as small but not negligible. Conclusions The results support the notion that IQ is a poor predictor of general aspects of everyday functioning in persons with mild/borderline intellectual disability. The result indicates that self‐ratings partly generate other information than proxy ratings which may be important for assessments of supportive requirements and diagnosis.