The disruption of chromatin modifiers during cortical neurogenesis is one of the underlying causes of neurodevelopmental and intellectual disability disorders. We recently found EZH1 loss and gain of function (LOF and GOF) variants as the genetic basis of overlapping novel neurodevelopmental disorders. Enhancer of Zeste Homologue 1 (EZH1), is one of the two histone H3 lysine 27 (H3K27) methyltransferases, and part of the Polycomb Repressive Complex 2 (PRC2). Its paralogue, EZH2, is essential in maintaining transcriptional repression of non-lineage specific genes during development. In neurodevelopment, EZH2 is highly expressed in dividing cells, and its dysfunction leads to defects in neural progenitor proliferation and fate specification. In contrast, the contribution of EZH1 to transcriptional silencing and neurodevelopment or disease is poorly understood. By mining publicly available transcriptomic datasets, we found that EZH1 is expressed in both the developing and adult nervous system, while EZH2 declines early during embryonic neurogenesis. These insights suggest a likely relevance of EZH1 in the developing and adult human brain and a causal role of EZH1 variants in the disease pathogenesis. To test this hypothesis, we generated neurodevelopmental models derived from EZH1 LOF and GOF human pluripotent stem cells (hPSC). Our monolayer neuronal differentiations showed that EZH1 LOF-derived neurons are less mature than wild-type neurons. Consistently, using cortical brain organoids, we uncovered that EZH1 is necessary for the coordination of cortical neurogenesis timing. Specifically, EZH1 LOF cortical organoids showed delayed neurogenesis of lower- and upper-layer projection neurons. In contrast, hyperactive GOF EZH1 neurons exhibited increased mature neuronal features, such as longer neurites. Furthermore, cortical organoids revealed that EZH1 GOF leads to premature neurogenesis specifically affecting upper-layer cortical neurons. Our work uncovers an essential role of EZH1 regulating cortical neurogenesis, and how its dysregulation leads to neurodevelopmental defects.