This paper presents the development of a mix design method for geopolymer mortar mixes with a principal focus on compressive strength. The method is based on the hypothesis that the overall strength of geopolymer mortar depends on the strength of the geopolymer binder materials, the packing density of the aggregates in the mortar and the amount of geopolymer binder. First, the strength of the binder was evaluated experimentally. Fly ash obtained from two different sources and ground granulated blast-furnace slag were used as the main ingredients in the production of the geopolymer binder. Correlation between the packing density of the aggregates in the mortar and their influence on the binder demand and compressive strength were investigated. Finally, a conceptual framework for a geopolymer mortar mix design approach is proposed. In addition to describing the mortar mix design methodology in a rational way, it is hypothesized that the approach can also be extended to the development of a geopolymer concrete mix.