The rheological and tribological performances of waste cooking oil (WCO) based novel sustainable grease with garlic oil as extreme pressure (EP) additive were examined according to the ASTM standards (ASTM D1092 and ASTM D2783) using a four-ball tester. The commercially available lithium-based grease served as reference grease for comparing and evaluating the rheological and tribological properties of the WCO-based greases. The addition of 6% garlic oil as an EP additive increased the weld load of WCO-based sustainable grease from 1560N to 2450N. The average wear scar diameter of WCO based and WCO + (GO + 6 wt.%) grease is reduced by 13.87% and 19.30% respectively, compared to that of commercial (MIDCO AP-3) grease. The results revealed that rheological properties of the WCO-based sustainable grease increase with the increase in the concentration of lithium soap. It is envisaged that garlic oil filled grease with WCO as base oil will not only have good lubricating properties but will also pose minimum environmental hazards.