Purpose: To evaluate the long-term oncological outcomes and toxicity profile based on 24 years of follow-up in patients with localized very high-risk prostate cancer (VHR PCa) treated with a combination of high-dose-rate brachytherapy (HDR-BT) and pelvic external beam radiation therapy (EBRT). Methods: A retrospective analysis was conducted on 87 patients with VHR PCa, classified according to National Comprehensive Cancer Network (NCCN) criteria, who received HDR-BT and EBRT. Androgen deprivation therapy (ADT) was administered to 72 patients (82.8%). The primary endpoints were biochemical control and cancer-specific survival (CSS), while the secondary endpoints included local control rates, tumor-free survival (TFS), overall survival (OS), and treatment-related toxicity. Results: The 24-year biochemical control rate was 68% (standard deviation [SD]: ±4%), while CSS and TFS at 24 years were 82% (SD ±4%) and 78% (SD ±4%), respectively. Local control rates remained at 98% at 24 years. Furthermore, the OS rate at 24 years was 30%. Multivariate Cox regression analysis identified the T category in the TNM classification as the only factor significantly associated with biochemical control, with 24-year rates of 69%, 71%, and 50% for patients with T-classifications of ≤T2c, T3a, and T3b-T4, respectively (p = 0.024). Notably, no grade ≥3 late toxicities were observed during the follow-up period. Conclusions: The 24-year outcomes support the viability and therapeutic efficacy of EBRT combined with a conformal HDR-BT boost for patients with VHR PCa.