Background/Objectives: Major cardiac surgery is associated with a high rate of postoperative complications, particularly in older and frail patients. Prehabilitation—defined as a preoperative intervention based on structured exercise—aims to enhance patients’ physiological and functional reserve before surgery. To evaluate the effectiveness of prehabilitation programs on functional capacity and postoperative complications in cardiac surgery and to propose a clinical exercise-based intervention model tailored to these patients. Methods: A systematic search was conducted in PubMed, Cochrane, PEDro, and LILACS (2005–2025). Randomized controlled trials investigating preoperative exercise interventions in adults undergoing cardiac surgery were included. Outcomes assessed included functional measures (6-Minute Walk Test [6MWT], Timed Up and Go test [TUG], maximal oxygen uptake [VO₂max], maximal inspiratory pressure [MIP]), frailty (Clinical Frailty Scale [CFS], Essential Frailty Toolset [EFT]), postoperative complications and quality of life. Results: Nine studies comprising a total of 873 patients were included. Prehabilitation significantly improved functional capacity (∆6MWT: +52.4 m; p < 0.001), reduced respiratory complications (pneumonia, atelectasis) and shortened hospital stay (−15.2 h; p < 0.001). The greatest benefits were observed in multimodal programs lasting ≥4 weeks. Conclusions: Exercise-based prehabilitation is an effective and safe strategy in patients awaiting cardiac surgery. Its systematic implementation should be considered as part of the perioperative pathway, reinforcing the role of exercise as a therapeutic tool in this clinical context.