Antifungal drug resistance in Aspergillus fumigatus is a major global health concern. Due to the emergence and increasing frequency of drug-resistant strains, there is a growing need for the development of novel antifungal drugs. Inhibition of the cysteine biosynthesis pathway in A. fumigatus represents a promising strategy for the development of antifungal adjuvants. Within this pathway, the previously uncharacterised enzyme AfuCys1a fulfils many desirable characteristics as an adjuvant target. To enable structure-based inhibitor design, AfuCys1a was structurally and functionally characterised using X-ray crystallography, biochemical assays, and comparative sequence and structural analysis. This research has provided insights into the catalytic activity of AfuCys1a, as well as key residues involved in substrate specificity, and identified candidate molecules as a structural basis for the development of antifungal drugs. In the search for effective and viable adjuvants, these findings can be used to narrow candidate inhibitors against AfuCys1a, working towards the larger goal of combating rising deaths from infection by A. fumigatus.