Pulsed cavity ringdown spectroscopy (CRDS) technique was used to develop a novel widely-tunable laser-based sensor for sensitive measurements of ethylene, propene, 1-butene and allene in the mid-IR. The use of an external-cavity quantum cascade laser (EC-QCL) enabled the sensor to cover a wide wavelength range from 10 to 11.1 µm (900 – 1000 cm-1) to detect multiple gases relevant to combustion and environment. The sensor operation was validated in a room-temperature static cell using well-characterized absorption lines of carbon dioxide near 938.69 cm-1 and 974.62 cm-1. Detection limits for ethylene, propene, 1-butene, and allene were measured to be 17, 134, 754 and 378 ppb, respectively, at 296 K and 760 Torr for a single-pass path-length of 70 cm. The excellent sensitivity of the optical sensor enabled it to measure the aforementioned gases at levels smaller than 1% of their recommended exposure limits. To the best of our knowledge, this is one of the first successful applications of the pulsed CRDS technique to measure trace levels of multiple gases in the 10 – 11 µm wavelength region.