Tryptophan C-mannosylation is an unusual metazoan protein modification that involves the formation of a carbon-carbon bond between the anomeric carbon of α-D-mannopyranose and C2 of tryptophan. The enzyme responsible for the modification, DPY19, is a multi-pass integral membrane protein found in the ER membrane that catalyzes the transfer of D-mannose from dolichol phosphate mannose (DPM) to tryptophan residues within the WXXW consensus sequence found in its client proteins. Although tryptophan mannosylation has been known for several decades, its biological roles remain unknown, mostly because it is difficult to produce pure protein samples with and without tryptophan C-mannosylation for functional studies. While several syntheses of C-mannosyltryptophan have been reported, these processes are lengthy and do not scale well, making it impractical to pursue the synthesis of pure glycoforms using solid phase synthesis and peptide-ligation technologies. To address this problem, we have developed a sophisticated photocatalytic system that enables the synthesis of C-mannosyltryptophan building blocks in high yield in just four linear steps from commercially available building blocks. This robust method has enabled multigram syntheses of C-mannosyltryptophan and the development of solution and solid-phase peptide synthesis methodologies.