A series of composite CaO-Ca₃Al₂O₆ mixed oxides were investigated as potential catalysts for biodiesel synthesis from waste fish oil. Different Ca/Al ratios, in the range of 1.5 to 6 were studied, alongside pure CaO. The catalysts were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and CO₂-Temperature Program Desorption (TPD). The catalytic activity of the materials was studied for the transesterification reaction of cod liver oil with methanol at 65 °C, with 1:12 oil to methanol molar ratio and 10 wt% of catalyst. Over 97% conversion of the triglycerides to methyl esters was achieved for the 6Ca/Al catalyst after 2 h reaction time. This was similar to the performance of CaO. However, 6Ca/Al catalyst was reused successfully for seven consecutive tests, in contrast to CaO that was reused for only five tests, before it deactivated. Therefore, by incorporating the Ca₃Al₂O₆, it was possible to enhance the stability of the catalytically active species and improve the lifetime of the catalyst. Post-test catalyst characterisation showed the formation of an intermediate phase (calcium diglyceroxide) that enhanced the catalyst’s performance and tolerance to air exposure and humidity. Finally, the catalyst deactivation, after seven cycles, took place due to the formation of Ca(OH)₂ and CaCO₃ species.