Dendritic cells (DCs) are armed with a multitude of Pattern Recognition Receptors (PRRs) to recognize pathogens and initiate pathogen-tailored T cell responses. In these responses, the maturation of DCs is key, as well as the production of cytokines that help to accomplish T cell responses. DC-SIGN is a frequently exploited PRR that can effectively be targeted with mannosylated antigens to enhance the induction of antigen-specific T cells. The natural O-mannosidic linkage is susceptible to enzymatic degradation, and its chemical sensitivity complicates the synthesis of mannosylated antigens. For this reason, (oligo)mannosides are generally introduced in a late stage of the antigen synthesis, requiring orthogonal conjugation handles for their attachment. To increase the stability of the mannosides and streamline the synthesis of mannosylated peptide antigens, we here describe the development of an acid-stable C-mannosyl lysine, which allows for the inline introduction of mannosides during solid-phase peptide synthesis (SPPS). By substituting the exo-cyclic oxygen with a methylene moiety, the mannosyl linkage is protected against acidic hydrolysis. The developed amino acid has been successfully used for the assembly of both small ligands and peptide antigen conjugates comprising an epitope of the gp100 melanoma-associated antigen and a TLR7 agonist for DC activation. The ligands showed similar internalization capacities and binding affinities as the O-mannosyl analogs. Moreover, the antigen conjugates were capable of inducing maturation, stimulating the secretion of pro-inflammatory cytokines, and providing enhanced gp100 presentation to CD8+ and CD4+ T cells, similar to their O-mannosyl counterparts. Our results demonstrate that the C-mannose lysine is a valuable building block for the generation of anti-cancer peptide-conjugate vaccine modalities.