Previous works on staghorn sumac (Rhus hirta) were mostly dedicated to its phytochemical profiles, antioxidant properties, and antidiabetic potentials. This study explored the potential of staghorn-sumac-derived functional ingredients for food and pharmacological applications. Sumac may have other biological functions, such as inhibitory effect on cancerous cells independent of its antioxidant properties. We characterized sumac and raspberry interactions, and their antioxidant capacities (ACs) and their inhibitory effect on both normal and cancerous cells. Mixing sumac and raspberry extracts yielded significantly higher ACs than the sum of sumac and raspberry as evaluated by three in vitro AC assays. However, the potential use of staghorn sumac as a natural source of dietary antioxidant supplement for oxidative-stress-related disorders might be challenged by its cytotoxicity in culturing normal cells. Remarkably, mixing sumac and raspberry showed maximal inhibition of the growth of both rat colon and human breast cancer cells with relatively low cytotoxicity toward normal rat colon and human breast epithelial cells, as compared with sumac or raspberry treatment alone. Sumac-derived products and their synergistic interactions with other food ingredients have great promise as functional food or nutraceutical products that would target cancer cells with minimal toxic effects to normal cells.