<p>Ultra-high-performance concrete combines exceptional strength and durability but carries a disproportionately high environmental burden due to its cement- and energy-intensive composition. Over the past decade, efforts to lower its carbon footprint have centred on incorporating supplementary cementitious materials, alternative binders such as low-carbon cements and nano-materials, and more recently, alkali-activated ultra-high-performance concrete systems. This review critically synthesises advances in low-carbon ultra-high-performance concrete, linking the chemical, mineralogical, and physical attributes of alternative constituents to hydration behaviour, microstructural evolution, and macroscopic performance. We highlight key microstructure–reactivity–performance relationships, evaluate sustainability trade-offs across diverse formulations, and identify the most viable pathways for reducing embodied carbon while maintaining mechanical performance. By integrating performance metrics with environmental assessments, this review provides clear guidance for the design, optimisation, and broader adoption of next-generation sustainable ultra-high-performance concrete.</p><p></p>