Mortar is one of the building materials that is widely used in most construction activities. So far, various methods have been used for designing the mixture proportions of mortar, and different ideas have been considered to improve the mortar’s quality. In this study, a novel densified mixture design algorithm (DMDA) developed by Hwang’s research group at the National Taiwan University of Science and Technology is applied for the mix design of the green mortar. The major concern of DMDA is to minimize the void volume of the aggregate system to obtain the maximum density of mortar with less water and less cement content, thus increasing the mechanical strength and durability performance of the mortar. In addition, the incorporation of both fly ash and ground granulated blast furnace slag in the DMDA method not only enhances the mortar’s quality but also provides a good solution for industrial waste treatment toward a cleaner environment and sustainable development. Another aim of this study is to investigate the changes in engineering properties and microstructure of the green mortars prepared with various lubricating paste contents (n). For this purpose, the green mortar samples were prepared with three different paste contents (n = 1.1, 1.3, and 1.5) and a fixed water/binder ratio of 0.4 was fixed for these mixtures. A reference mix designed by the conventional method was also included for comparison purpose. In the fresh stage, the flow diameter of all fresh mortar mixtures was adjusted in the range of 180 ± 5 mm by using various superplasticizer dosages and the fresh unit weight of these mixtures was also checked. In the hardened stage, the mortar samples were subjected to various tests including flexural strength, compressive strength, water absorption, and drying shrinkage. Moreover, a scanning electron microscopy technique was also used to observe the microstructure of the mortars. The experimental results show that increasing the paste content resulted in an increase in flexural strength, compressive strength, and drying shrinkage of the green mortars, meanwhile, a reduction in their fresh unit weight was found. Besides, all of the mortar samples exhibited a denser microstructure with low water absorption rates. A maximum compressive strength value of about 53 MPa could be achieved for the green mortar sample at 28 days with n = 1.5. It could also be observed from the results that all of the green mortars exhibited better performance than the reference mortar. The results of this study further demonstrated the effectiveness of using DMDA for the mix design of green mortars for sustainable construction.