Background: Rehabilitation of the posterior atrophic maxilla frequently requires simultaneous management of vertical and horizontal bone deficiencies. While lateral sinus floor elevation remains the most predictable approach for severe vertical atrophy, reconstruction of associated horizontal defects often requires additional bone augmentation procedures, increasing surgical complexity and morbidity. The use of the lateral sinus wall (“sinus bone window”) as an autologous cortical membrane may provide a biologically driven alternative for simultaneous three-dimensional reconstruction. Objective: To evaluate the clinical and radiographic outcomes of a sandwich grafting technique using the sinus bone window as an autologous cortical membrane combined with particulate autologous bone and plasma rich in growth factors (PRGF-Endoret®) for the treatment of mixed posterior maxillary atrophy. Materials and Methods: A retrospective case series was performed including patients with combined vertical and horizontal posterior maxillary atrophy treated by lateral sinus floor elevation and simultaneous horizontal ridge augmentation using the sinus bone window as a cortical membrane. Clinical records and CBCT scans were reviewed. Horizontal bone gain, implant survival, prosthetic survival, peri-implant marginal bone loss, and complications were analyzed. Statistical significance was established at p < 0.05. Results: Sixteen patients (81.3% women; mean age 61.2 ± 11.8 years) with 20 regenerated posterior maxillary sites were included. Initial ridge width increased from 2.11 ± 1.52 mm to 8.46 ± 1.65 mm, representing a mean horizontal gain of 5.35 ± 1.75 mm (172.6% increase; p < 0.001). All regenerated sites achieved sufficient width for implant placement without additional augmentation procedures. Twenty narrow-diameter implants were placed, achieving a mean insertion torque of 34.71 ± 18.25 Ncm. After a mean follow-up of 80.5 ± 3.6 months, implant and prosthetic survival rates were 100%. No surgical complications were recorded. Mean marginal bone loss was 0.73 ± 0.18 mm mesially and 0.85 ± 0.34 mm distally. Conclusions: The use of the sinus bone window as an autologous cortical membrane appears to be a predictable technique for simultaneous horizontal and vertical reconstruction of the posterior maxilla. This approach provides substantial horizontal bone gain, avoids a secondary donor site, facilitates implant placement in severely atrophic ridges, and demonstrates excellent long-term clinical and radiographic stability.