There is a bidirectional and continuous relationship and communication between the brain and the gut. The gut-brain axis communicates not only through the microbiota but also via the autonomic nervous system. Microorganisms in the gut microbiota are known to contribute to the production of certain neurotransmitters that play roles in brain function, which may, in turn, influence mental health. Through this interaction, the gut microbiota and gastrointestinal system components play significant roles in human health. A healthy functioning of the gut-brain axis is essential for supporting the immune system, regulating mood, and maintaining cognitive and immune health. Recent studies highlight a connection and causality between the gut-brain-microbiota axis and neurological disorders such as Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, and autism spectrum disorder, as well as psychiatric disorders like depression, anxiety, schizophrenia, and bipolar disorder. This paper reviews the impact of the gut-brain-microbiota axis on brain and mental health, emotion-thought-behavior systems, and its effects on neurological and psychiatric disorders.