Significance Heteromannans are ancient plant cell wall polysaccharides, ubiquitous throughout the plant kingdom. In plants, these hemicellulosic polymers strengthen the cell wall and facilitate energy storage, and are involved in cell expansion and developmental signaling. Heteromannans are used extensively by humans, as healthy dietary fibers and as thickening agents in food, cosmetic, pharmaceutical, and mining industries. The fine structure of heteromannan is important for the polysaccharide’s biophysical properties, and thus its function and application. However, the biosynthesis of different heteromannan structures remains mechanistically unclear. We demonstrate that plant mannan and glucomannan can be synthesized heterologously in the yeast Pichia pastoris , and the polysaccharide’s structure can be modulated by specific plant cofactors, allowing for the synthesis of tailored mannan structures.