Clay is a material that is rich in oxides of aluminum and silicon, which are also abundant in fly ash, and have strong potential to be used as a precursor for the synthesis of a sustainable geopolymer green brick. This experimental work was designed to optimize the amount of clay and fly ash as precursors. Eleven mix proportions were designed, the clay was replaced by fly ash from 0 to 100 wt%, At macro-scale, the physical and mechanical properties of resultant geopolymer bricks were assessed and compared with the codes for a conventional fired clay brick. It was observed that the brick with 30 to 60 wt% fly ash having; low density, allowable water absorption and high compressive strength can be used as a load bearing material. At micro level, the geopolymerization was assessed by Fourier transform infrared (FTIR) spectroscopy, which was also verified by the anorthite peak in the X-ray diffraction (XRD) analysis. The XRD results are complimented with the FTIR and verify the gain in mechanical strength of geopolymer bricks by polycondensation. The resultant geopolymer green brick is a sustainable construction material with smooth manufacturing process along with superior physical and mechanical properties as compared to a conventional brick.