Due to the increased demands of adapting the sustainability concept in the construction industry, many researchers have developed and evaluated the composite materials made with agricultural by-products, such as straws, fruit-shells, and cobs, as construction materials. Because no research work has been reported regarding the incorporation of common reed fiber (CRF) into a concrete composite to produce the green and sustainable concrete, this research has focused on the evaluation of physical, mechanical, and thermal properties of mortar mixture containing CRF regarding density, porosity, compressive and flexural strengths, and thermal conductivity. In total, six mixtures with 0%, 2%, 4%, and 6% CRF; 0.5% steel fiber (SF); and the combination of 6% CRF and 0.5% SF were prepared. Based on the experimental outputs, a simple analysis of heat loss was also been performed. The test results presented that the incorporation of CRF into mortar mixture proportionally reduced its unit weight and significantly increased its absorption capacity and porosity. Although the use of only CRF in the mortar mixture did not improve both compressive and flexural strengths compared to the plain mixture, the combined use of CRF and SF to increase both compressive and flexural strengths generated a synergetic effect to increase both strengths. The addition of CRF to the mixture has the benefit of producing a significant decrease in heat loss for a typical building in Astana due to the lower thermal conductivity and higher porosity to density ratio.