Lysine demethylases (KDMs) play a pivotal role in the establishment of cell fate and their loss or reduction has been linked to neurodevelopmental disorders and intellectual disability (ID). Kdm1a and Kdm5c are two of the most expressed KDMs in hippocampal neurons. The two enzymes converge in maintaining a repressive state, consequently, the ablation of either one of them in adult hippocampal excitatory neurons triggered the derepression of non-neuronal genes in neurons. To explore the functional interaction between the two proteins we generated and characterized double inducible and forebrain-restricted knockouts. The combined depletion exacerbated the transcriptional and epigenetic dysregulations observed in single mutants, unveiling the role of both enzymes safeguarding the topological boundaries of a subset of polycomb complex repressed genes. Although the individual elimination of either protein in the adult brain only caused mild behavioral alterations, their combined ablation led to more severe cognitive impairments, including spatial memory deficits and impaired fear conditioning, and alterations in neuronal excitability in hippocampal CA1 pyramidal neurons. These results underscore the convergent role of these ID-linked epigenetic regulating cell-type specific gene silencing, fine-tuning H3K4 methylation levels, and preserving chromatin organization and cognitive function in adult neurons.