1,2,3-Trichlorobenzene (TCB) was ground with calcium oxide (CaO) powder in air with a planetary ball mill. A mechanochemical reaction was induced, resulting in the decomposition of TCB through dechlorination from the benzene nucleus. The mechanochemical dechlorination was monitored by a suite of analytical methods including X-ray diffraction (XRD), thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, electron spin resonance (ESR), gas chromatography/mass spectrometry (GUMS), and ion chromatography. With the increase in grinding time, the remaining amount of TCB decreased and reached 0.02% at 360 min grinding. On the other hand, the water-soluble amount of chlorine increased correspondingly and reached 95%, indicating the successful transformation of chlorine from organics into inorganic chloride. Additionally, the other final products detected after grinding were carbon and some minor methane and ethane, although the formation of some chlorine-containing intermediate phases such as dichlorobenzene and tetrachlorobenzene was observed in the early stage of grinding (for I h). A possible decomposition pathway based on dechlorination or dehydrochlorination is compared.