The aim of this study was to document the natural history of development and long-term progression of osteoarthritis (OA) in the feline knee after minimally invasive anterior cruciate ligament (ACL) transection. ACL transections of the left knee joint of 14 skeletally mature cats were performed. Radiographic scores, tibiofemoral and patellofemoral joint space and anterior tibial translation were assessed before, immediately and every 3 months after ACL transection (longest follow-up: 93 months). After 26 months, all ACL transected knees had developed definite OA. The earliest changes were observed on the tibia plateau starting as early as 2 months after ACL transection, and at 12 months signs of OA were present in more than 80% of cats in the medial and in almost 80% of cats in the lateral compartment. In the first 24 months, medial tibiofemoral joint space decreased by 0.88 mm (95% confidence interval −0.55;−1.21 mm) and lateral tibiofemoral joint space by 0.55 mm (−0.26;−0.85 mm). In the same interval, the joint space in the patellofemoral joint increased by 0.98 mm (0.59; 1.37 mm). Throughout the entire observation period, the anterior tibial translation was on average 5.3 mm greater than in the contralateral knee (4.5; 6.0mm). Immediate changes in anterior tibial translation during an anterior drawer test clearly showed joint instability that persisted throughout the lifetime of the animals. Degenerative changes were observed on radiographs within 4 months of the injury only in the transected but not the contralateral limb suggesting the role of mechanical instability for the development and progression of knee OA.