Laser filamentation overcomes diffraction over a highly extended distance, making itself a powerful tool for long-range stand-off detection and light detection and ranging (LIDAR) applications. Mid-infrared (mid-IR) wavelengths are optimal for detecting biochemicals and air pollutants due to molecular fingerprints. Here, we demonstrate mid-IR laser filamentations in ambient air at a kilohertz repetition rate for the first time. Laser filaments significantly longer than the linear confocal parameter are generated with a pump power exceeding the critical power in air using a kilohertz, 2.1 μm, femtosecond, multi-millijoule optical parametric chirped-pulse amplifier. Odd-harmonic generation up to the ninth order at ultraviolet and the mid-IR spectral extensions up to 3.5 μm are observed. The highest third and fifth harmonic efficiencies from ambient air are obtained, to our knowledge, thanks to the extended interaction length within the filaments. Numerical simulations reproduce the harmonic generation with good agreement and confirm that the plasma effect dominates over the higherorder Kerr effect as the main defocusing mechanism of laser filamentation in our experiment. The detection of atmospheric CO 2 is demonstrated via mid-IR absorption spectroscopy. High-flux ultrabroadband mid-IR filaments are useful for the fast and sensitive detection of multiple chemical species in air.Laser filamentation is a highly nonlinear process that occurs as the self-focusing from the optical Kerr effect is balanced by diffraction, plasma defocusing, and other nonlinear mechanisms. The laser peak power needs to be higher than the critical power that scales with λ 2 to reach the filamentation regime, where λ is the laser wavelength. Filamentation has been widely studied in solids and high-pressure gases for various applications, such as supercontinuum generation, few-cycle pulse compression, and high-harmonic generation [1,2]. Filamentation in air has especially attracted considerable attention for applications in remote sensing, LIDAR, and laser-induced breakdown spectroscopy [3,4]. Compared to the traditional LIDAR and spectroscopy based on linear laser beam propagation, filamentation overcomes diffraction over much longer distances, and, additionally, the back scattered light is much more directional [2]. This makes laser filamentation a powerful tool for long-range stand-off detection. Moreover, the broadband supercontinuum from the filamentation covers a large spectral range, enabling us to access multiple single-wavelength traces simultaneously in a single-shot measurement. However, most experimental work on filamentation in ambient air has been limited to the visible and near-infrared (near-IR) range, where high-energy, ultrafast laser sources are readily available. By extending the spectral range to the mid-IR, one could explore higher-order nonlinear processes, as the odd harmonics in the visible and near-IR regions propagate well in the atmosphere through filaments [5][6][7]. More importantly, mid-IR filamentation in air ...