Quality of life (QoL) is a key outcome used to monitor service planning and delivery for individuals with Intellectual and Developmental Disabilities (IDD). Unfortunately, many current instruments used to measure QoL have psychometric and content limitations and none are suitable for use with individuals with the lowest levels of functioning and highest support needs. This study assessed the reliability and validity of English translations of the San Martín (n = 236) and KidsLife (n = 93) scales with individuals with concurrent diagnoses of Autism and IDD. All participants were between the ages of 17 and 52 years old (M = 30.7, SD = 8.51) and required full-time residential care. Scales were completed by educational (n = 39) and residential (n = 92) staff who had known the participants for a minimum of six months. Temporal stability (r = 0.73-0.97) and inter-language consistency (r = 0.65-0.97) was established for the San Martín translation. The KidsLife translation had strong interrater reliability (r = 0.74-0.93). Predictive validity between the two scales was low across domains; however, evidence of discriminant validity suggested that the English translations of the San Martín (p = 0.09-0.94) and KidsLife (p = 0.08-0.93) scales measured a separate construct from intelligence. Results suggest that English translations of the San Martín and KidsLife provide reliable and valid estimates of QoL for individuals with concurrent Autism and IDD and moderate to profound support needs. Implications of these findings and guidelines for use of these instruments are discussed.