© The Author(s) 2020.1,3-Bis-(2-methyl-2-propenyl)benzimidazolium chloride and its Ag(I) complex are synthesized and the structures are elucidated using spectroscopies techniques. The molecular and crystal structures of the benzimidazolium salt are confirmed by X-ray crystallography. The molecular geometries of the benzimidazolium and its Ag(I) salt are analyzed using the B3LYP functional with the 6–311+G(d,p)/LANL2DZ basis set. The observed Fourier transform infrared and nuclear magnetic resonance isotropic shifts are compared with the calculated values. Besides, the quantum chemical identifiers, significant intramolecular interactions, and molecular electrostatic potential plots are used to show the tendency/site of the chemical reactivity behavior. The three-dimensional Hirshfeld surfaces and the associated two-dimensional fingerprint plots are applied to obtain an insight into the behavior of the interactions in the crystal. Both compounds are tested for their in vitro anticancer activities against DU-145 and MCF-7 cancer cells and L-929 non-cancer cells using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay.