While cancer immunotherapies have become central to treatment, challenges associated with the ability of tumors to evade the immune system remain significant obstacles. At the heart of this issue is the tumor immune microenvironment, the complex interplay of the tumor microenvironment and the immune response. Recent advances in mRNA cancer vaccines represent major progress towards overcoming some of the challenges posed by deleterious components of the tumor immune microenvironment. Indeed, major breakthroughs in mRNA vaccine technology, such as the use of replacement nucleotides and lipid nanoparticle delivery, led to the vital success of mRNA vaccine technology in fighting COVID-19. This has in turn generated massive additional interest and investment in the platform. In this review, we detail recent research in the nature of the tumor immune microenvironment and in mRNA cancer vaccines and discuss applications by which mRNA cancer vaccines, often in combination with various adjuvants, represent major areas of potential in overcoming tumor immune microenvironment-imposed obstacles. To this end, we also review current mRNA cancer vaccine clinical trials.