Non-marketable postharvest residue from Satsuma Owari mandarins (Citrus unshiu)—discarded due to having a puffy peel (MC) or being damaged (MD)—was evaluated against a commercial mature reference (MM) using an integrated cascade biorefinery. Along the liquid pathway, must fermentation (6.1–9.0% v/v) and fractional distillation yielded a citrus brandy (~30% v/v). Vinasse emerged as the stream of highest strategic relevance, maximizing bioactive response (up to 1339.1 µg GAE/mL) with a naringenin-dominated phytochemical profile. Along the solid pathway (peel), sequential recovery of essential oil and pectins was performed. Essential oil yields (0.575–0.85 mL/100) decreased significantly with physical peel damage, underlining flavedo integrity. Concurrently, recovered pectins (18.6–23.3%) exhibited a degradation-driven structural transition: MM retained galacturonic acid-rich linear domains (89.8%), whereas in MD they had evolved toward a branched architecture with higher rhamnogalacturonan I (30.6%). All fractions were low-methoxyl (DM < 50%), ensuring functionality in ionic gelling systems. In conclusion, this reproducible strategy successfully transforms postharvest losses into functional ingredients and chemical distillates, providing a robust foundation for circular economy models within the citrus industry.