The present study investigates the effects of the alkyl chain length of the cationic head-group of some ammonium-based ionic liquids (ILs) (having the same anion) on the interaction between the ILs and N,N-dimethylacetamide (DMA). The molecular interactions between the studied ILs, tetraethylammonium hydroxide (TEAR), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), and their binary mixtures with DMA were studied using the Fourier transform infrared spectroscopy (FTIR) technique, dynamic light scattering (DLS) experiments, and quantum chemical calculations. It was observed from both experimental FTIR analysis and theoretical studies that the strength of intermolecular interactions, such as hydrogen bonding, ion-ion interactions, and induced dipole interactions, between the ILs and DMA depends on the alkyl chain length of the IL cation head-group. The interaction of DMA with IL is energetically favorable and occurs via direct interactions between the IL anion and the carbonyl oxygen of DMA. The results further revealed that the shorter the alkyl chain length of the cationic head-group of the ILs, the stronger the interaction with the DMA molecule, such that the strength of interactions between the ILs and DMA follows the order TEAR > TPAH > TBAH. This trend can be attributed to the increased self-organized aggregation with increasing alkyl chain length of the IL cation.