Summary Mannans with β‐1,4‐linked backbones are common cell wall components of algae and land plants. Prior challenges to enhance β‐mannan content in plants point to unclear metabolic bottlenecks and the potential for hidden biosynthetic players. Endo‐β‐MANNANASEs (MANs) are the main glycosyl hydrolases that mobilize extracellular β‐mannans during seed germination. However, we found that Arabidopsis man2 man5 seeds resemble β‐mannan biosynthetic mutants. CELLULOSE SYNTHASE‐LIKE A (CSLA) overexpression restored β‐mannan synthesis in the man double mutant and increased the distribution of crystalline polymers but impaired the release of other mucilaginous polysaccharides. Using yeast synthetic biology, we dissected the functional interplay of MAN enzymes with CSLAs. Intracellular MAN2 and MAN5 reduced the quantity of insoluble β‐mannan but elevated the content of water‐soluble carbohydrates. We propose that Arabidopsis MAN2/5 and orthologous crop enzymes with a transmembrane domain sustain hemicellulose production in the Golgi apparatus by cleaving insoluble β‐mannan polymers into hydrophilic counterparts.