Background: The p53 tumor suppressor is mutated in more than half of all cancers. COTI-2, an oral 3rd-generation thiosemicarbazone, restores the structure and function of mutant p53 proteins and inhibits growth of p53 mutant cancer cell lines. While few anticancer agents are effective as monotherapies in the clinic in part due to resistance, the COTI-2 mechanism of action to reactivate normal p53 function complements DNA-damaging radiation- or chemo-therapy. Our data demonstrate that COTI-2 has a promising role in combination treatments inducing synergistic anticancer responses. Methods: Cell viability was measured with crystal violet stains or indicator dye to assess IC₅₀. Xenograft studies used cancer cell lines with specified p53 mutations. Drug synergy was determined by combination-index and isobolograms generated using CalcuSyn software. Results: COTI-2 and chemotherapy agents were tested, singly and in combination, in carcinoma cell lines: colorectal, non-small cell lung (NSCLC), and head and neck squamous cell (HNSCC). Cell lines had wild-type p53, a variety of p53 mutations, or unknown status. COTI-2 IC₅₀ for the HNSCC lines, both established and early passaged, were 9.6-370.0 nM with most p53 mutants having lower IC₅₀ than wild-type. In other work, single agent and combination treatment with COTI-2 and approved anticancer agents in colorectal carcinoma, NSCLC, or HNSCC cells in vitro showed strong synergism against mutant p53 cell lines. In mice bearing A2780 or PCI13 tumors with mutant p53, COTI-2 and doxorubicin or cisplatin, singly and in combination, demonstrated significant enhancement of tumor growth inhibition, and combinations were well tolerated. Lastly, COTI-2 and radiotherapy were administered singly and in combination in a PCI13 tumor model and showed strong synergy against tumor growth. Conclusions: COTI-2 is a highly potent, orally bioavailable compound active against many tumors with p53 mutations. This work highlights the potential of combination treatment using COTI-2 with other anticancer agents to provide synergistic tumor growth inhibition with good tolerability.