As an emerging alternative to the conventional ordinary Portland cement (OPC) binder, it is imperative to maximise the efficiency of clinker-free and low carbon geopolymer composites under normal as well as harsh conditions such as sulfate /seawater exposure, freeze-thaw, high temperatures, etc. Including fibers in brittle and quasi-brittle materials is a well-known approach to improve the flexural strength, energy absorption capacity and cracking behaviour under imposed, thermal and shrinkage loads. In this study, the impacts of steel fiber reinforcement on the fresh (flow), mechanical and durability properties of 100% ground granulated blast furnace slag (GGBFS) based geopolymer are investigated for different fiber volume fractions, fiber lengths, alkaline monomer ratios and curing regimes. The mixes are produced with steel fiber volume fractions varying at 0, 1.25 and 2.5%; fiber length varying at 6 and 12 mm; alkaline activating solution monomer ratio varying at 2.5 and 3.0, followed by adoption of ambient as well as heat curing regimes. Initially, the workability, density, compressive and flexural strength of mixes are evaluated in a routine manner under different curing regimes and then the composite mixes are exposed to sulfate environment, sea water, freeze-thaw and high temperatures of 150?,300?, 600?, and 800? to evaluate the residual weight, compressive strengths and ultrasonic pulse velocity (UVP) values. The microscopic morphology of the fiber-matrix interfacial transition zone is characterized using field emission scanning electron microscope (FESEM) and x-ray diffractometer (XRD). Test results confirmed that additions of steel fibers significantly improve both the compressive and flexural strength, besides enormously improving the durability properties with regards to fire resistance, sulfate resistance and freezethaw resistance. The ambient cured fibrous GGBFS geopolymer composites have shown immense potential for in-situ construction purposes.