Neurodevelopmental biology, coupled with the application of advanced histological, imaging, molecular, cellular, biochemical, and genetic approaches, has provided new insights into these intricate genetic, cellular, and molecular events. During telencephalic development, specific neural progenitor cells (NPCs) proliferate, differentiate into numerous cell types, migrate to their apposite positions, and form an integrated circuitry. Critical disturbance to this dynamic process via genetic and environmental risk can cause neurological disorders and disability. The phosphatidylinositol-3-OH kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling cascade contributes to mediate various cellular processes, including cell proliferation and growth, and nutrient uptake. In light of its critical function, dysregulation of this node has been regarded as a root cause of several neurodevelopmental diseases, such as megalencephaly (“big brain”), microcephaly (“small brain”), autism spectrum disorders, intellectual disability, schizophrenia, and epilepsy. In this review, particular emphasis will be given to the PI3K-Akt-mTOR signaling pathway and their paramount importance in neurodevelopment of the cerebral neocortex, because of its critical roles in complex cognition, emotional regulation, language, and behaviors.