Protein glycosylation plays a pivotal role in regulating protein folding, stability, and secretion. Dysregulation of glycosylation has been implicated in a wide range of diseases. Among the less common types of glycosylation, C-mannosylation—defined by the covalent attachment of a mannose residue to the indole C2 carbon of tryptophan via a carbon–carbon bond—remains relatively poorly characterized. In this review, we comprehensively examine the landscape of C-mannosylation across diverse proteins, identifying previously unrecognized C-mannosylated substrates and elucidating the functional consequences of this modification, including its impact on protein secretion and enzymatic activity. We further characterize DPY19L3, a multi-pass endoplasmic reticulum membrane protein, as a bona fide C-mannosyltransferase. Our findings delineate its enzymatic activity, membrane topology, and functional involvement in myogenic differentiation and tumor cell vasculogenic mimicry. In the course of identifying novel C-mannosylated proteins, mass spectrometry analyses of CCN1 and FGL1 revealed the presence of another atypical glycosylation—glucosyl-galactosyl-hydroxylation (GGH)—a modification typically observed in collagen-like domains. Notably, neither CCN1 nor FGL1 contains such domains. Functional studies using GLT25D1 and LH3 knockout models confirmed that galactosylation and subsequent glucosylation of hydroxylated lysines in FGL1 are sequentially catalyzed by these enzymes. Collectively, our findings expand the current understanding of atypical glycosylation, establish mechanistic links between specific glycosyltransferases and protein function, and highlight potential therapeutic avenues targeting C-mannosylation and GGH in pathological contexts.