Polysaccharides are the most abundant biopolymers on earth that serve various structural and modulatory functions. Pure, completely defined linear and branched polysaccharides are essential to understand carbohydrate structure and function. Polysaccharide isolation provides heterogeneous mixtures, while heroic efforts were required to complete chemical and/or enzymatic syntheses of polysaccharides as long 92-mers. Here, we show that automated glycan assembly (AGA) enables access to a 100-mer polysaccharide via a 201-step synthesis within 188 h. Convergent block coupling of 30- and 31-mer oligosaccharide fragments, prepared by AGA, yielded a multiple-branched 151-mer polymannoside. Quick access to polysaccharides provides the basis for future material science applications of carbohydrates.