C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. P. falciparum gametogenesis was imaged in its entirety in four dimensions using lattice light-sheet microscopy. This revealed defects in egress and exflagellation for DPY19 microgametes. While exflagellation was diminished, DPY19 microgametes still fertilized macrogametes, forming ookinetes but these were abrogated for mosquito infection. The gametogenesis defects corresponded with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect was concordant with defective CTRP secretion on the DPY19 background. Genetic complementation of DPY19 restored ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 in the early secretory pathway ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.