The dire need for sustainable construction materials has resulted in emerging research to improve the properties and, subsequently, the structural performance of the geopolymer composite. One of these progressive moves is this study’s focus on enhancing the mechanical properties of geopolymer composite. This experiment employed a unique methodology in preparing pristine graphene-reinforced geopolymer mortar. Moreover, the study’s successful dispersion of a large-size (50 µm) industrially manufactured pristine graphene (PG) and its effect when incorporated in the geopolymer matrix was the first of its kind in research on geopolymer. The dosages of PG by weight of the precursor added to the geopolymer mix were 0.05%, 0.07%, 0.1%, and 0.3%. The results revealed that PG less than 5% by weight of the dispersing medium produced a good dispersion when sonicated in an aqueous solution and polycarboxylate ether superplasticiser as a surfactant. An ultraviolet-visible spectrophotometer was used to affirm that the PG aqueous solution subjected to ultrasonication was stable, well dispersed, and fit for incorporation in the geopolymer mortar. When the 0.07% dosage of the PG was incorporated in the geopolymer mortar, the compressive strength was highest, reaching 61.2 MPa and 63.5 MPa at 7 and 28 days, respectively. At 28 days after adding the 0.07% dosage of PG to the geopolymer mortar, the direct tensile strength was maximum at 2.5 MPa, while the flexural strength had a maximum of 10.4 MPa. An optimum PG dosage of 0.07% significantly improved the compressive, tensile, and flexural strengths by 14.4%, 25.96% and 17.35% at 28 days, respectively. Furthermore, the hypothesis tested acknowledged that the 0.05% and 0.07% PG dosages were responsible for significant improvement of the mechanical properties of the geopolymer mortar. This study affirms that large-size industrially produced PG could revolutionise the entrant of sustainable construction materials.