The research focused on the development of an ultra-lightweight cementitious composite with both excellent mechanical and thermal insulating properties. Fly ash cenosphere (FAC), and aerogel, a nano-structured highly porous material made of silica, were used as lightweight aggregates. Polyvinyl alcohol fibers were used to improve the mechanical behavior of the cementitious composite. The experimental results showed higher specific strength (up to 18 kPa/kg m) of the resulting composites as compared to conventional lightweight materials. Depending on the amount of FAC and aerogel, the compressive and flexural strengths of the cementitious composite were found as 23.54-18.63 MPa and 4.94-3.66 MPa, respectively, while the thermal conductivity was reduced to 0.3197 W/m-K. Moreover, the hydration products and microstructures of the FAC/aerogel modified cementitious composite were investigated by the Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (EDS). Thermal stability of the hardened matrix was studied by using thermo-gravimetric analyses and it was revealed that the composites were fairly stable at a high temperature range. The weight loss varied with increasing aerogel content. In conclusion, both FAC and aerogel are excellent candidates for producing mechanically strong as well as thermally insulated composites which have great potential to be used in buildings for energy conservation.