The transesterification of food oils to biodiesel under various ratios of magnesium slag to MeOH was investigated in terms of fatty acid methyl ester (FAME) yield. The use of slag as a heterogeneous catalyst enabled the conversion of used cooking oil into biodiesel. The optimal conditions for transesterification using untreated magnesium slag were studied, and it was found that maximum conversion was 96% after 12 h of reaction. The maximum achievable FAME yield was observed at a ratio of untreated magnesium slag to oil of 15 to 20% and a methanol (MeOH) to oil ratio of 20%. To investigate the mechanism of the transesterification reaction, the heterogeneous solid catalyst MgO-CaO/Al2O3, derived from magnesium slag was also tested. Maximum FAME conversion (98%) was achieved in 30 min, and it was noted that the MgO-CaO/Al2O3 catalyst showed increased performance in the presence of CO2. The experimental work indicated that temperature is the main driving force for the transesterification reaction, leading to a FAME conversion efficiency of 98-99% within 1 min in the temperature range of 350 to 50 degrees C. In summary, the use of the environmentally benign magnesium slag is a potential substitute for the commercially used homogeneous acid/base catalyst.