The social determinants of intellectual disability (ID) are poorly understood, particularly in Australia. This study has investigated sociodemographic correlates of ID of unknown cause in Western Australian born children. Using record linkage to the Western Australian Maternal & Child Health Research Database, maternal sociodemographic characteristics of children with ID (of unknown cause) born between 1983 and 1992 (n = 2871) were compared with those of children without ID (n = 236,964). Socioeconomic indices for areas based on the census district of mother's residence were also included in the analysis. Aboriginal mothers (OR = 2.83 [CI: 2.52, 3.18]), teenagers (OR = 2.09 [CI: 1.82, 2.40]) and single mothers (OR = 2.18 [CI: 1.97, 2.42]) were all at increased risk of having a child with mild or moderate ID. Children of mothers in the most socioeconomically disadvantaged 10% had more than five times the risk of mild and moderate ID compared with those in the least disadvantaged 10% (OR = 5.61 [CI: 4.42, 7.12]). Fourth or later born children were also at increased risk (OR = 1.82 [CI: 1.63, 2.02]). The results of the study have implications both for further aetiological investigation as well as service provision for children with ID. Furthermore, many of the sociodemographic correlates identified in this study, particularly in the mild/moderate category of ID, are potentially modifiable, opening up opportunities for primary prevention.