Mathematics is an important aspect of daily life. Basic numeracy skills are needed to accomplish everyday tasks. However, research regarding the relationship between cognitive ability, mental age, and basic numeracy skills for children with intellectual disability (ID) is scarce. This research study investigated the correlation between intellectual ability and the development of number sense and arithmetic abilities in individuals with ID. Thirty-two students from a special school for individuals with ID in Singapore participated in this research study. Based on the results, basic numeracy is more closely correlated to the mental ages of the students than their intelligence quotient. This finding will be discussed and suggestions for future research will be provided.