Investigators have increasingly turned to studying the retina as a window into brain structure and function. In neuropsychiatric diseases, retinal anatomy as assessed by optical coherence tomography (OCT) and retinal cell function as assessed by various forms of electroretinography (ERG) demonstrate notable changes. In addition, many studies indicate significant correlations between retinal changes and clinical features such as cognitive decline, overall illness severity, and progression of illness. Here, we review retinal findings in psychiatric (schizophrenia, autism, mood disorders, attention deficit hyperactivity disorder, anorexia nervosa), and neurologic (multiple sclerosis, Parkinson's disease, Alzheimer's disease and mild cognitive impairment, Huntington's disease, traumatic brain injury) conditions, in terms of their potential as biomarkers of disease onset, progression, severity, and outcomes. Consistency and variability in findings across studies are highlighted, and implications for future research are discussed. Potential confounds and methodological issues central to studies of retinal structure and function in neuropsychiatry are also considered. The review concludes with discussions of: a) recent advances in retinal imaging and their potential applications for studying brain disorders; and b) the potential for applications of artificial intelligence to increasing the predictive validity of retinal data.