Transcranial electrical stimulation (tES) is often used in clinical practice as an adjunct to other treatments. However, it is still unclear how the combination of different methods affects the overall effectiveness of therapy and how effective tES is compared to other therapeutic approaches. A search and systematic review of studies according to the PRISMA criteria were conducted in the PubMed database up to April 4, 2025. At the primary search stage, the following articles were included: articles on the treatment of mental disorders using various types of tES as part of combination therapy or compared to therapy with psychopharmacological agents, psychotherapy methods, and cognitive training. One thousand six hundred eighty-nine publications were retrieved, of which 46 were included in the review. The anodic tES of the left dorsolateral prefrontal cortex (DLPFC) remains the most studied tES method. Its effectiveness in depression is likely to be comparable to antidepressants in medium therapeutic dosages. The augmentation of anodic tES of the left DLPFC with antidepressant therapy, but not cognitive-behavioral therapy, also demonstrated an advantage over monotherapy in non-resistant depression. Anodic tES of the left DLPFC combined with cognitive training in mild cognitive impairment is probably more effective than either method alone. There are a few studies both on individual nosologies and on other types of tES. The limitation of many studies is a small sample size. Therefore, TES is a promising therapeutic approach that can take its place in the treatment of mental disorders; however, further studies of the diversity of TES approaches are required both in different nosologies and in various combinations with other treatment methods.