This paper aims to find the optimum proportions of alkali-activated mortar mixtures made with waste perlite powder (PP) and granulated blast furnace slag (BFS) for superior fresh and mechanical performance. The PP-BFS blended mortar mixtures were developed using the Taguchi method. Four factors, including the binder content (500, 550, and 600 kg/m3), PP replacement percentage by BFS (25, 50, and 75%), alkali-activator solution-to-binder (S/B) ratio (0.60, 0.65, and 0.70), and sodium silicate-to-sodium hydroxide (SS/SH) ratio (1.0, 1.5, and 2.0), were considered in the design phase. Accordingly, an L9-sized orthogonal array was developed, leading to a total of nine alkali-activated mortar mixtures. The target design criteria were the flow and 7-day compressive strength. The analysis of variance (ANOVA) revealed that the S/B and SS/SH ratios contributed the most to the workability of mixes. Conversely, the PP replacement by BFS was the controlling factor among others on the 7-day compressive strength. Using the Taguchi method, the optimum mix proportions for superior flow ability were binder content of 500 kg/m3, WPP replacement percentage by BFS of 25%, S/B ratio of 0.70, and SS/SH ratio of 1.5. Meanwhile, the highest strength response was attained while using a binder content of 600 kg/m3, WPP replacement percentage by BFS of 25%, S/B ratio of 0.6, and SS/SH ratio of 2.0.