Background: Mild Cognitive Impairment (MCI) is seen as a state between normal aging and dementia, with patients having an increased risk of developing Alzheimer’s disease (AD) and other sorts of dementia. MCI has been linked to a change in gut microbiota which impacts the microbiota-gut-brain axis (MGBA), consequently affecting neurological functions. A way of altering microbiota and thereby promoting cognitive health is through the administration of prebiotics, probiotics, and synbiotics. Aim: This systematic literature review aims to assess the impact of prebiotics, probiotics, and synbiotics on MCI by compiling existing data on the matter. Methods: Three databases - Web of Science, Cochrane, and PubMed - were searched and articles were included based on the following inclusion criteria: (1) randomized clinical trials (RCTs), (2) conducted on adults evaluated with MCI during the study, (3) including a prebiotic, probiotic, or synbiotic intervention of any kind, (4) comparing the intervention with a placebo or control group, (5) written in English, (6) reporting the main outcome of cognitive function using any neuropsychological evaluation test. Results: Five studies were included in the final selection. These studies showed that cognitive function improved after probiotic intervention, significantly affecting several cognitive domains: attention, calculation, orientation in time, and delayed memory. Two studies showed that subjects with low cognitive scores at baseline benefited more from probiotic supplementation compared to high-scoring subjects. Conclusions: Probiotics appear to improve cognition in MCI subjects; however, further research is needed to conclude the effects of prebiotics and synbiotics.