(1) Background: Megaprosthetic reconstruction of the knee is frequently required in limb salvage surgery for oncologic indications and in complex revision arthroplasty. The Modular Universal Tumor and Revision System (MUTARS) Total Knee prosthesis is widely used in these situations. The aim of this study was to evaluate revision-free implant survival following MUTARS Total Knee implantation and to compare outcomes between primary implantation and use as a conversion procedure after failure of a previous knee prosthesis. (2) Methods: A retrospective cohort study was performed including 36 patients who underwent MUTARS Total Knee implantation at a single institution. Patients were stratified into primary implantation (n = 24) and conversion after failed prosthesis (n = 12). The primary endpoint was time to first revision. (3) Results: Overall revision-free survival was 51.6% at 2 years and 29.3% at 5 years. No significant difference in revision-free survival was observed between primary implantation and conversion procedures (log-rank p = 0.67). When mechanical failure (Henderson type III) was considered as the sole endpoint, implant survival was substantially higher, with 88.3% survival at 2 years and 57.2% at 5 years. Infection-related implant survival was 72.0% at both 2 and 5 years. Secondary amputation was required in 7 of 36 patients (19.4%), with a higher proportion observed in the primary implantation group (25.0% vs. 8.3%). Most amputations occurred within the early postoperative period. (4) Conclusions: MUTARS Total Knee implantation is associated with a substantial revision burden; however, conversion after failed prosthesis did not result in inferior revision-free survival compared with primary implantation. Endpoint-specific analyses demonstrate that biological complications, particularly infection, represent the dominant drivers of early failure.