This study examined changes in mental fitness before and after a music therapy program for middle-aged and older men residing in facilities. A total of 12 music therapy sessions were conducted with 15 participants, and data from the final 12, excluding dropouts, were analyzed. A single-group pre-post design was applied to analyze changes in the total mental fitness score and its subfactors: mental energy, empathic communication, flexibility, and self-assurance. The Wilcoxon signed-rank test showed statistically significant increases in the total mental fitness score and the flexibility subfactor after the program. Flexibility had a large effect size. These results suggest the clinical usefulness of music therapy as a non-pharmacological intervention to activate psychological adaptation resources and enhance mental fitness in this population. This study is significant in that it analyzed the mental fitness of middle-aged and older men residing in facilities from a multidimensional perspective, providing a baseline for follow-up research, including a control group.