Isoniazid (INH) is an antitubercular drug that exhibits high toxicity in dogs due to the absence of N-acetyltransferase activity in this species. Consequently, it has been implicated in both accidental and intentional poisonings in dogs. The aim of this study was to develop and validate an analytical method for the quantification of INH in canine liver samples and to apply it in the forensic investigation of seven suspected poisoning cases. The method, based on liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (LC-QTOF-MS), enabled both accurate INH measurement and analysis of the molecular pattern of its metabolite formation. In addition, histopathological examination of the stomach, pancreas, liver, and brain was performed. Liver INH concentrations ranged from 11.822 to 30.484 μg/g and were associated with extensive necrotic lesions across all examined tissues. A strong signal for isonicotinic acid was observed in all samples, whereas the acetylated metabolite was negligible. The developed method allows precise quantification of INH in canine liver and facilitates identification of the characteristic molecular profile of its metabolites.