Background/Objectives: Combining autologous fat grafting with implant placement is meant to improve the quality of implant-based breast reconstruction. The present study explores the concept of multi-stage composite breast reconstruction with repeated sessions of autologous fat grafting to increase mastectomy flap thickness and provide better pre-pectoral implant coverage. Methods: Twenty-five consecutive patients underwent bilateral multi-stage composite expander-to-implant breast reconstruction and reverse expansion from August 2020 to April 2024. Subcutaneous thickness of the mastectomy flap was evaluated in predefined regions of interests of the breast on standardized MR images at two timepoints (before the first fat grafting session, with the tissue expander fully inflated, and 3 months after implant placement). Furthermore, the incidence of complications requiring surgery and implant-related complications were evaluated. All values are expressed as mean ± standard deviation, accepting statistical significance for a p-value < 0.05. Results: Patients underwent an average of 2.5 ± 0.6 fat grafting sessions, with a fat injection volume of 170 ± 60 mL per breast per session. The mean duration of the reconstructive process from mastectomy to final implant placement was 12 ± 5 months and the mean follow-up was 17 ± 8 months. The overall thickness of both breasts amounted to 190% of baseline thickness and was significantly higher in the upper breast quadrants than in the lower quadrants (p < 0.05). Tissue thickness increase correlated well with the number of fat grafting sessions and was independent of the patient's weight gain. Complications requiring surgery occurred in eight breasts during the reconstruction, with iatrogenic expander puncture being the most frequent (three cases, 6%). During follow-up, only one implant-related complication was observed (one case of bilateral rippling, 4%). No breast animation or symptomatic capsular contracture were observed. Conclusions: Multi-stage pre-pectoral composite expander-to-implant breast reconstruction using autologous fat grafting is an effective concept for breast reconstruction. Despite the need for multiple surgeries, the significant increase in subcutaneous tissue thickness, resulting in better soft tissue coverage, compensates for the longer reconstructive process.