A quantum cascade laser (QCL)-based absorption sensor for the simultaneous dual-species monitoring of CH4 and N2O was developed using a novel compact multipass gas cell (MGC). This sensor uses a thermoelectrically cooled, continuous wave, distributed feedback QCL operating at ~7.8 µm. The QCL wavelength was scanned over two neighboring CH4 (1275.04 cm-1) and N2O (1274.61 cm-1) lines at a 1 Hz repetition rate. Wavelength modulation spectroscopy ( f = 10 kHz) with second harmonic (2 f) detection was performed to enhance the signal-to-noise ratio. An ultra-compact MGC (16.9 cm long and a 225 ml sampling volume) was utilized to achieve an effective optical path length of 57.6 m. With such a sensor configuration, a detection limit of 5.9 ppb for CH4 and 2.6 ppb for N2O was achieved, respectively, at 1-s averaging time.