Cellular immunotherapy is an important treatment that uses a patient’s own immune cells to fight cancer and other diseases. In this therapy, the immune cells (usually T cells) can be engineered to express chimeric antigen receptor (CAR) proteins that recognize cancer cells. However, the success of CAR-related therapy is often limited by immune cell death or immunosuppression in the tumor environment. One option is to use mesenchymal stem cells (MSCs), found in various tissues and body fluids, to supplement or replace the immune cells in CAR-related therapy. MSCs can regulate immune cells through direct interactions or by secreting anti-inflammatory molecules and growth factors. MSCs also release exosomes—small vesicles containing DNA, RNA, proteins, and lipids—to affect target cells. When used in conjunction with CAR-modified cells, MSCs can modulate the proliferation and anticancer functions of those cells, and engineered CAR-expressing MSCs have been shown to be able to target and kill glioblastoma and Ewing’s sarcoma cells. Notably, MSCs can promote tumor growth in some situations, so further research is necessary to ensure that the right MSCs are selected for specific applications. Nevertheless, the existing evidence suggests that MSCs are promising tools for improving CAR-related cellular immunotherapy.