Fungal-type galactomannan (FTGM) is a polysaccharide composed of α-(1 → 2)-/α-(1 → 6)-mannosyl and β-(1 → 5)-/β-(1 → 6)-galactofuranosyl residues located at the outer cell wall of the human pathogenic fungus Aspergillus fumigatus. FTGM contains a linear α-mannan structure called core-mannan composed of 9 or 10 α-(1 → 2)-mannotetraose units jointed by α-(1 → 6)-linkages. However, the enzymes involved in core-mannan biosynthesis remain unknown. We speculated that two putative α-1,2-mannosyltransferase genes in A. fumigatus, Afu5g02740/AFUB_051270 (here termed core-mannan synthase A [CmsA]) and Afu5g12160/AFUB_059750 (CmsB) are involved in FTGM core-mannan biosynthesis. We constructed recombinant proteins for CmsA and detected robust mannosyltransferase activity using the chemically synthesized substrate p-nitrophenyl α-D-mannopyranoside as an acceptor. Analyses of CmsA enzymatic product revealed that CmsA possesses the capacity to transfer a mannopyranoside to the C-2 position of α-mannose. CmsA could also transfer a mannose residue to α-(1 → 2)-mannobiose and α-(1 → 6)-mannobiose and showed a 31-fold higher specific activity toward α-(1 → 6)-mannobiose than toward α-(1 → 2)-mannobiose. Proton nuclear magnetic resonance (1H-NMR) spectroscopy and gel filtration chromatography of isolated FTGM revealed that core-mannan structures were drastically altered and shortened in disruptant A. fumigatus strains ∆cmsA, ∆cmsB, and ∆cmsA∆cmsB. Disruption of cmsA or cmsB resulted in severely repressed hyphal extension, abnormal branching hyphae, formation of a balloon structure in hyphae, and decreased conidia formation. The normal wild type core-mannan structure and developmental phenotype were restored by the complementation of cmsA and cmsB in the corresponding disruptant strains. These findings indicate that both CmsA, an α-1,2-mannosyltransferase, and CmsB, a putative mannosyltransferase, are involved in FTGM biosynthesis.