A series of 3,3-diethylazetidine-2,4-dione based thiazoles 3a–3j were designed and synthesized as new human neutrophil elastase (HNE) inhibitors in nanomolar range. The representative compounds 3c, 3e, and 3h exhibit high HNE inhibitory activity with IC50 values of 35.02–44.59 nM, with mixed mechanism of action. Additionally, the most active compounds 3c and 3e demonstrate high stability under physiological conditions. The molecular docking study showed good correlation of the binding energies with the IC50 values, suggesting that the inhibition properties are largely dependent on the stage of ligand alignment in the binding cavity. The inhibition properties are correlated with the energy level of substrates of the reaction of ligand with Ser195. Moreover, most compounds showed high and broad-spectrum antiproliferative activity against human leukemia (MV4-11), human lung carcinoma (A549), human breast adenocarcinoma (MDA-MB-231), and urinary bladder carcinoma (UMUC-3), with IC50 values of 4.59–9.86 μM. Additionally, compounds 3c and 3e can induce cell cycle arrest at the G2/M phase and apoptosis via caspase-3 activation, leading to inhibition of A549 cell proliferation. These findings suggest that these new types of drugs could be used to treat cancer and other diseases in which immunoreactive HNE is produced.