Objectives: The main objective of the study was to compare mortality for people with an intellectual disability (ID) to the general population in New South Wales (NSW), Australia. A second objective was to provide mortality data for people with an intellectual disability from NSW in a standardized format, which allows for international comparisons of the mortality of people with an intellectual disability. Design and setting: The study was a longitudinal cohort study linking retrospective data from the population databases of the NSW Department of Ageing, Disability and Home Care (ADHC) Intellectual Disability Services Dataset (intellectual disability cohort) to the mortality data set of the NSW Registry of Births, Deaths and Marriages (RBDM) for the period 1 June 2005 to 31 December 2011. Participants: The two groups of interest for this study were persons with an intellectual disability recorded in the intellectual disability cohort (953 deaths among 42 219 persons over 270 649 person-years of observation) and the rest of the NSW population (312 649 deaths among over 7 218 529 persons [2011 population] and 45 818 946 person-years of observation). Main outcome measures: In this study, we compared the intellectual disability cohort with the rest of the NSW population using age-adjusted death rates (ADRs), standardized mortality ratios (SMRs) and age standardized death rates (ASDRs) based upon the World Health Organization (WHO) standard population. Results: We found a significantly higher mortality in a cohort of people with an intellectual disability compared to the rest of the NSW population. The intellectual disability cohort compared to the rest of the NSW population had an SMR of 2.48 for all ages and an SMR of 3.15 for those aged 5-69 years. We also found an SMR for females (4.26) with an intellectual disability that was significantly higher than that for males (2.52) with an intellectual disability. The ASDR for the intellectual disability cohort was 4.04 (deaths per 1000) and the ASDR for the rest of the NSW population was 1.58, giving a comparative mortality ratio of 2.55. Conclusions: Our findings indicate excess mortality of people with an intellectual disability when compared to the general population. They also indicate that females with an intellectual disability have a higher relative mortality compared to female same-aged general population peers, and also males with an intellectual disability, a finding replicated in other similar cohort studies. For international comparison using the WHO standard population, our intellectual disability cohort (ages 5-69 years) had a mortality of 4.04 deaths per 1000.