Cement production contributes to carbon dioxide emissions in the construction industry. Sustainable alternative materials are required to address the environmental impacts of cement production as partial replacement. Hence, to reduce natural resource consumption and decrease the environmental impact, agro-industrial wastes can be added to cementitious composites. This research paper investigates to produce sustainable cement in the form of cement mortar. The cement mortar of 1:3 (cement: fine aggregate) mix used for the present study. The activated charcoal powder (ACP) was produced from Eucalyptus bark which is agricultural waste. The industrial by-product fly ash (FA) was procured from nearby thermal power plant. SEM and EDX analyses revealed chemical composition of ACP and FA, respectively. The cement mortar cubes of standard size specimens were casted with ACP and FA ranging from 0% to 5% and 0% to 30% of cement weight, respectively, as partial replacement. The water cured mortar cubes were tested for compressive strength at 3, 7, 14, and 28 days as per standard procedure. The cement mortar samples with 4% ACP and 20% FA as replacement materials for cement resulted in to 18.63% and 12.26% increase in compressive strength, respectively, comparing with conventional mortar at 28 days. The pozzolanic interaction between ACP and FA with cement matrix resulted in higher strength of cement mortar. Further increase in percentage of ACP and FA as replacement for cement reduced the compressive strength. Hence, partial replacement of cement by ACP and FA in cement mortar provides a cost-effective alternative and benefits environment by reducing CO2 emissions.