A tridentate Schiff base ligand, (E)-3-((2-hydroxy-3-methoxybenzylidene)amino)-2-methylquinazolin-4(3H)-one [HL], and its mixed-ligand Ni(II) complex [Ni(L)(imi)], were synthesized and fully characterized using elemental analysis, FT-IR, UV-Vis and 1 HNMR spectroscopy techniques. The structure of the synthesized ligand and complex was determined with single crystal X-ray diffraction method. In the complex, a square planner geometry was observed around the Ni(II) central atom coordinated with the donor atoms of the Schiff base ligand and one nitrogen of imidazole group. In addition, the catalytic activity of the complex on the three-component condensation of hydrazine hydrate with phthalic anhydride and dimedone to obtain 2H-indazolo[2,1-b]phthalazine-triones was investigated. Furthermore, in-vitro antimicrobial studies were performed that indicated the great antibacterial activities of the Ni(II) complex against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Bacillus cereus bacteria.