High expression of the immunosuppressive enzyme, indoleamine 2,3-dioxygenase 1 (IDO1) for a broad range of malignancies is associated with poor patient prognosis, and the enzyme is a validated target for cancer intervention. To identify novel IDO1 inhibitors suitable for drug development, 1597 compounds in the National Cancer Institute Diversity Set III library were tested for inhibitory activity against recombinant human IDO1. We retrieved 35 hits that inhibited IDO1 activity >50% at 20 μM. Five structural filters and the PubChem Bioassay database were used to guide the selection of five inhibitors with IC50 between 3 and 12 μM for subsequent experimental evaluation. A pyrimidinone scaffold emerged as being the most promising. It showed excellent cell penetration, negligible cytotoxicity and passed four out of the five structural filters applied. To evaluate the importance of Ser167 and Cys129 residues in the IDO1 active site for inhibitor binding, the entire NCI library was subsequently screened against alanine-replacement mutant enzymes of these two residues. The results established that Ser167 but not Cys129 is important for inhibitory activity of a broad range of IDO1 inhibitors. Structure-activity-relationship studies proposed substituents interacting with Ser167 on four investigated IDO1 inhibitors. Three of these four Ser167 interactions associated with an increased IDO1 inhibition and were correctly predicted by molecular docking supporting Ser167 as an important mediator of potency for IDO1 inhibitors.