We demonstrate herein a facile synthesis of a series of highly-ordered low-melting protic ionic liquids with linear C₁₆–C₂₀ side chains identical to natural fatty acids, which were produced by the S-alkylation of the 2-mercapto-1-methylimidazole (methimazole) ring. There is considerable structural latitude possible when designing highly lipophilic protic ionic liquids that exhibit low T_m values due to the cumulative disruptive effects of thioether and unsaturation on their packing efficiency. Their bulk thermophysical properties, hydrophobicity and cytotoxicity were also determined.