In this study, fourteen new herbicidal ionic liquids (HILs) based on MCPB with a 1-alkyl-1-methylpiperidinium cation were synthesized and characterized. The effect of the alkyl chain length on the physicochemical properties of HILs was determined. Additionally, thermal analysis, solubility in 10 representative solvents, and surface and herbicidal activity were determined. The herbicidal efficacy was tested in greenhouse experiments by using common lambsquarters (Chenopodium album L.) and oilseed rape (Brassica napus L.) as test plants. The ionic liquids were more effective than the commercial herbicide. Moreover, there was a relationship between the surface activity of HILs and herbicidal efficiency.