This study uses densified mixture design algorithm (DMDA) to design the mixture proportions green mortars incorporating fly ash and ground granulated blast furnace slag (GGBFS). Three green mortar mixtures with various paste contents were designed using the DMDA method while a control cement-based mortar mixture was designed using the conventional method for comparison. The effect of different paste contents (n = 1.1, 1.3, and 1.5) on the changes of porosity, thermal conductivity, compressive strength, chloride ion penetration, resistance to sulfate attack, and ultrasonic pulse velocity (UPV) of both the green and control mortars was studied. The procedures of DMDA mix design for green mortar mixtures were also presented in this study. Test results reveal that all three green mixtures show a better performance than the control mixture, especially for long-term properties. On the other hand, increasing paste content increased porosity and compressive strength, however, it reduced thermal conductivity and UPV of the green mortars. All of the green mortars exhibited good performance with low porosity and compressive strength values of above 40 MPa after 28 days of curing. Moreover, the green mortars also demonstrated excellent ability to resist chloride (with RCPT values significantly lower than the threshold of 1000 coulombs) and sulfate attacks. As a result, the quality of these mortars was classified as very good level with UPV values of above 4000 m/s. This study proves the effectiveness of using the DMDA method to design the green mortar mixtures in the combination of using fly ash and GGBFS.