Joint injury is the predominant risk factor for post‐traumatic osteoarthritis development (PTOA). Several non‐invasive mouse models mimicking human PTOA investigate molecular mechanisms of disease development; none have characterized the inflammatory response to this acute traumatic injury. Our aim was to characterize the early inflammatory phase and later degenerative component in our in vivo non‐invasive murine model of PTOA induced by anterior cruciate ligament (ACL) rupture. Right knees of 12‐week‐old C57Bl6 mice were placed in flexion at a 30° offset position and subjected to a single compressive load (12N, 1.4 mm/s) to induce ACL rupture with no obvious damage to surrounding tissues. Tissue was harvested 4 h post‐injury and on days 3, 14, and 21; contralateral left knees served as controls. Histological, immunohistochemical, and gene analyzes were performed to evaluate inflammatory and degenerative changes. Immunohistochemistry revealed time‐dependent expression of mature (F4/80 positive) and inflammatory (CD11b positive) macrophage populations within the sub‐synovial infiltrate, developing osteophytes, and inflammation surrounding the ACL in response to injury. Up‐regulation of genes encoding acute pro‐inflammatory markers, inducible nitric oxide synthase, interleukin‐6 and interleukin‐17, and the matrix degrading enzymes, ADAMTS‐4 and MMP3 was detected in femoral cartilage, concomitant with extensive cartilage damage and bone remodelling over 21‐days post‐injury. Our non‐invasive model describes pathologically distinct phases of the disease, increasing our understanding of inflammatory episodes, the tissues/cells producing inflammatory mediators and the early molecular changes in the joint, thereby defining the early phenotype of PTOA. This knowledge will guide appropriate interventions to delay or arrest disease progression following joint injury. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:2118–2127, 2018.