Seed mucilage is a natural hydrophilic polysaccharide gel secreted by seed coat epidermal cells comprising xylan, pectin, glucomannan, and cellulose alongside minor proteins, minerals, and lipids, with its composition varying substantially across plant species. This review critically compares conventional and green extraction technologies, including ultrasound-, microwave-, enzyme-, and subcritical water-assisted extraction, on a seed-mass-normalized yield basis together with their solvent-to-seed ratios and processing times, showing that green technologies can reduce solvent consumption, energy use, and processing time relative to matched conventional controls, though the magnitude of these gains is method and species-specific rather than uniform. We synthesize how molecular weight, uronic acid content, branching pattern, and microfibril organization govern seed mucilage hydration, rheological, emulsifying, and foaming behavior, and link these structure–function relationships to its antioxidant, prebiotic, anti-inflammatory, and antimicrobial bioactivities. Beyond functional characterization, this review evaluates the translational barriers, extraction standardization, safety and regulatory status, and clinical validation that currently limit seed mucilage adoption in food, pharmaceutical, and cosmetic industries. Unlike previous reviews focused on ecological function or application cataloging, this review uniquely reframes seed mucilage as a structurally programmable bioactive polysaccharide, integrating molecular architecture, green process intensification, and structure–function–application relationships to bridge fundamental chemistry with industrial translation.