The field of peptide-mediated cargo delivery has seen an evolution in the last four decades, starting with the naturally derived transactivator of transcription (TAT) and penetratin cell-penetrating peptides (CPPs) and gradually evolving to rationally designed peptides for specific cargos. Early work in the field focused on oligonucleotide CPP delivery but has lately pivoted to gene editors, such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9. CPP-mediated Cas9 delivery has great potential as a cancer treatment to treat the underlying cause of the disease and/or disrupt cancer-specific cellular pathways. The combination of CRISPR/Cas9 with conventional chemotherapy is especially exciting. Furthermore, novel peptide designs can create activatable peptides that are capable of responding to the tumor microenvironment and thereby reducing off-target effects of the peptide and the CRISPR/Cas9 system simultaneously. The work in this field is still in its early stages, but results are promising, and progress is being made. Currently, the main application of peptides in cancer is the use of targeting peptides to increase cargo delivery to the tumor, but research into other peptide applications is progressing. Lastly, developments in peptide-mediated gene editor delivery into immune cells are promising to enhance cell-mediated cancer treatments, such as augmented chimeric antigen receptor (CAR) T cells. Thus, the future of peptide-mediated CRISPR/Cas9 delivery to tumors is exciting and promising. However, much work is still required in the field, such as showing the clinical benefit of CPP-mediated CRISPR/Cas9 delivery to cells.