The Activity-Dependent Neuroprotective Protein (ADNP) is an important regulator of early brain development, especially during cortical neurogenesis and neurite formation. De novo point mutations or haploinsufficiency of the ADNP gene result in ADNP syndrome, which is also known as Helsmoortel-Van der Aa syndrome, a complex neurodevelopmental disorder recognized as a leading single-gene cause of syndromic autism spectrum disorder (ASD) and intellectual disability. ADNP works as both a transcription factor and a microtubule (MT) regulator. As a transcription factor, ADNP is a key component of chromatin remodeling complexes such as ChAHP (CHD4 (Chromodomain Helicase DNA-binding Protein 4)-ADNP-HP1 (Heterochromatin Protein 1)) and SWI/SNF (Switch/Sucrose Non-Fermentable), and it tightly regulates the expression of numerous essential developmental genes. ADNP also modulates the Wnt/β-catenin signaling pathway. During neural differentiation, ADNP is redistributed from the nucleus to the cytoplasm, and this redistribution is regulated by binding to 14-3-3 proteins, which are phosphorylated by protein Kinase C (PKC). After relocating to the cytoplasm, ADNP functions as an MT regulator by binding to microtubule end-binding proteins (EB1 and EB3) and Tau to control neurite formation. Previous studies have focused on NAP (also known as Davunetide, a peptide derived from ADNP) in MT regulation and its therapeutic potential for autism spectrum disorder (ASD) and neurodegenerative diseases, such as Alzheimer’s disease. This review highlights the functions of full-length ADNP and NAP in early brain development, particularly in neurogenesis and neurite formation during cortical development. We will also discuss the potential of NAP as a therapeutic medication for neurodevelopmental disorders, especially ASD and ADNP syndrome.