During the development of the mammalian cerebral cortex, appropriate numbers of neurons, glial cells, and oligodendrocytes must be generated and functionally integrated to form appropriate neuronal circuitry. The regulation of gene expression by DNA-binding transcription factors is crucial for this development, and abnormal brain development can result in intellectual disability. The transcription factor RP58 has been reported to regulate cerebral cortical development by suppressing the transcription of target genes. Human genetic association studies have recognised the importance of RP58 for human neuronal development, with genetic mutations to RP58 associated with abnormal brain development and intellectual disability in humans. However, the causative nature of genetic mutations to RP58 remains to be clarified. This study investigates the possible pathological consequences of two de novo, missense mutations to RP58 (N461S and R495G), detected in two unrelated patients diagnosed with intellectual disability. Immunolocalisation studies revealed that the subcellular localisation of the two variants differs to the wild-type protein. Strikingly, a luciferase reporter assay revealed a unique outcome for the R495G variant, which exhibited transcriptional activation rather than repression. The N461S variant was also observed to disrupt the transcriptional regulatory activity of RP58. Finally, in utero electroporation experiments show that both variants have different capacities to restore the defective migration of Rp58 shRNA-treated cells. Together, these findings demonstrate that these disease-associated mutations alter the transcriptional regulatory function of RP58, and impair its capacity to control radial migration during cerebral cortex development.