An 11-year-old male castrated domestic shorthair cat was presented for evaluation due to clinical deterioration and potential extrahepatic biliary obstruction (EHBO). Further investigations confirmed EHBO and revealed severe and previously unreported comorbidities. On initial examination, the cat was markedly icteric with a poor body condition score and severe muscle wasting. Serum chemistry and complete blood count showed evidence of cholestasis and anemia. Primary diagnostics and therapeutics targeted these abnormalities. Abdominal ultrasound revealed peritoneal effusion, multifocal mixed echogenic hepatic and splenic foci, small intestinal thickening, cholelithiasis, choledocholithiasis, and common bile duct and pancreatic duct dilation with evidence of obstruction. Peritoneal effusion cytology confirmed septic peritonitis. Hepatic and splenic cytology was consistent with lymphoma. Based on these results, euthanasia was elected by the owners of the animal. Necropsy confirmed the ultrasound diagnoses, septic peritoneal effusion associated with a duodenal perforation, multiorgan lymphoma, and common bile duct carcinoma. Flow cytometry classified the lymphoma as a double-negative phenotype of T-cell lymphoma (CD3+ and CD5+, but CD4- and CD8-) present in the duodenum and liver and suspected in the spleen which has previously not been reported in cats. This case report documents a cat with EHBO caused by multiple disease processes including a novel T-cell lymphoma phenotype, biliary carcinoma, duodenal perforation and septic abdomen, and choleliths, as well as inflammatory hepatobiliary disease.