Benzene (C6H6) is one of the major public health concerns. It is emitted from various natural and anthropogenic sources, like fires and volcanic emissions, petrol service stations, transportation, and the plastics industry. Here, we present our work on developing a new benzene sensor using a widely tunable difference-frequency-generation (DFG) laser emitting between 11.56 and 15 µm (667–865 cm–1). The DFG process was realized between an external-cavity quantum-cascade-laser and a CO2 gas laser in a nonlinear, orientation-patterned GaAs crystal. We obtained the absorption cross-sections of the Q-branch of the ν4 vibrational band of benzene by tuning the wavelength of the DFG laser between 14.79 and 14.93 μm (670–676 cm–1). Benzene sensing measurements were performed near 14.84 µm (673.97 cm–1) with a direct laser absorption spectroscopy scheme. The benzene concentration was varied between ppb and ppm levels. Even with a relatively short optical path-length of 23 cm, our sensor achieved a benzene detection limit of about 10 ppb.