Being a relatively newer material in construction sector, geopolymer based building materials have been of large interest for last decade. This study presents a comprehensive experimental study for assessment of the effectiveness of mix parameters and compound characteristics of the ingredients on the engineering properties of lightweight geopolymer mortar (LWGM). The investigated fresh properties are rheology and flow behavior of LWGMs. Mechanical properties of hardened LWGMs were monitored in terms of compressive and flexural strength as well as ultrasonic pulse velocity. Acidic pumice aggregate was used to reduce the unit weight of the mortar. The mix parameters taken into account are molarity of NaOH solution, the mass ratio of NaOH and Na2SiO3 solutions to fly ash (S/FA), proportions of Na2SiO3/NaOH (SS/SH), and the oxide compositions, which are percentage of Na2O, the ratios of H2O/Na2O, SiO2/Na2O, and H2O/Solid. The results indicated that SS/SH and S/FA significantly affected viscosity behavior and the fresh mortars were observed to conform well to Herscel-Bulkley rheological model with shear thinning tendency. The mechanical properties of hardened LWGMs demonstrated strong correlation with oxide compositions.