Lincomycin A is a potent antimicrobial agent noted for its unusual C1 methylmercapto-substituted 8-carbon sugar. Despite its long clinical history for the treatment of Gram-positive infections, the biosynthesis of the C-8-sugar, methylthiolincosamide (MTL), is poorly understood. Here, we report our studies of the two initial enzymatic steps in the MTL biosynthetic pathway leading to the identification of D-erythro-D-gluco-octose 8-phosphate as a key intermediate. Our experiments demonstrate that this intermediate is formed via a transaldol reaction catalyzed by LmbR using D-fructose 6-phosphate or D-sedoheptulose 7-phosphate as the C-3 donor and D-ribose 5-phosphate as the C-5 acceptor. Subsequent 1,2-isomerization catalyzed by LmbN converts the resulting 2-keto C-8-sugar (octulose 8-phosphate) to octose 8-phosphate. These results provide, for the first time, in vitro evidence for the biosynthetic origin of the C-8 backbone of MTL.