Purpose: To prospectively assess long-term (20 year) clinical, radiographic, and patient-reported outcomes of an elderly population provided with mandibular implant-supported overdentures. Materials and Methods: A total of 53 elderly (aged ≥60 years at the time of treatment) were provided with two endosseous implants supporting a mandibular overdenture and a conventional maxillary denture. Outcome parameters—including implant loss, plaque index, gingival index, bleeding index, presence of calculus, probing depth, and satisfaction with implant-supported overdenture—were scored 1, 5, 10, and 20 years after prosthetic treatment. Radiographic analysis was performed to assess peri-implant bone changes. At the 20-year evaluation, frailty (Groningen Frailty Index) and quality of life (EuroQol 5D) were additionally assessed. Results: A total of 15 patients completed the 20-year follow-up. The 20-year implant survival rate was 92.5%. Plaque index, bleeding index, and probing depth increased slightly over time, while gingival index and presence of calculus remained unchanged. Radiographic analysis revealed minor marginal bone loss during the first 10 years and no further loss thereafter. Participants were very satisfied with their prosthesis and reported a good quality of life. At the 20-year evaluation, 64.3% of the patients were classified as frail. Conclusions: The long-term survival of implants supporting a mandibular overdenture is high. Although most elderly in the study became frail over time, peri-implant health and marginal bone level remained at a satisfactory level.