Introduction: The first aim of this study was to explore the mental functions of patients with mild intellectual disabilities or borderline intellectual functioning (MID-BIF) in mental health care. The second aim was to examine the differences in mental functions between people with MID-BIF and those with higher educational levels (i.e. further education [FE] and higher education [HE]). Materials and methods: Eleven mental functions were assessed using the Shedler-Westen Assessment Procedure (SWAP-200-NL). In total, 245 patients were included, divided into three groups (MID-BIF: N = 52; FE: N = 86; HE: N = 107). Results: Results showed that the MID-BIF group scored relatively high on three mental functions (i.e. the capacity for impulse control and regulation, capacity for defensive functioning, and capacity to construct and use internal standards and ideals). One mental function scored relatively low (the capacity for adaptation, resiliency, and strength). Regarding group comparisons, similar patterns in average scores emerged, and the HE group scored higher than the MID-BIF and the FE groups on most mental functions. Discussion: The mental functions of the HE group were most adaptive, whereas the mental functions of the FE and MID-BIF groups were less adaptive and on a similar level in each of these groups.