The emergence and dissemination of high-risk clones of producing extended-spectrum β-lactamases (ESBLs) in animal infections is a critical issue. We report the detection and genomic features of a multidrug-resistant (MDR) ESBL (CTX-M-15)-producing infecting a domestic cat. Whole-genome sequencing analysis identified the international ST340 (clonal group CG258), and genes and mutations conferring resistance to β-lactams, aminoglycosides, macrolides, phenicols, fosfomycin, sulfonamides, tetracycline, trimethoprim, and fluoroquinolones. In addition, the presence of genes encoding resistance to disinfectant and heavy metals hazardous to humans was also confirmed. The MDR profile exhibited by the strain contributed to treatment failure and death of the companion animal. Therefore, active surveillance of critical priority lineages of should not only focus on human infections but also on veterinary infections.