Extraction of oil from wild mustard seeds with a yield of 34.0% w/w, followed by transforming it to biodiesel fuel via KOH-catalyzed transesterification with methanol assisted by a co-solvent with applying dry washing for purifying the as-produced methylic esters using the activated carbon which was prepared from mustard de-oiled residue was the major target of this investigation. Impact of reaction parameters on co-solvent transesterification of wild mustard oil (WMO) with methanol, like the molar ratio of methanol/WMO, amount and kind of co-solvent, the KOH amount, co-solvent/methanol % v/v, temperature and the duration, were inspected. The excellent conditions that resulted in the production of maximum biodiesel yield (97.88 ± 1.50%, 96.89 ± 1.0%purity) was accomplished using 6:1 methanol/WMO molar ratio, 1:1 hexane/methanol (% v/v), 0.80% KOH w/w, a temperature of 50°C for 45 minutes and (4.0% w/w) of the activated carbon for purifying the obtained unpurified methylic esters. Properties of the as-produced biodiesel were analyzed in accordance with ASTM D6751 typical test methods and found fulfilled the biodiesel typical criteria. The 1H NMR and FTIR spectroscopy analysis guaranteed transformation of the WMO to methylic esters. It was concluded that co-solvent methanolysis is more effective, time saving and helpful as it could lessen the optimal conditions necessary for attaining the conversion compared with the usual process.