With the continuous expansion of the lithium industry, lithium slag (LS) has been generated in large quantities, and its potential reuse in cement-based materials has become increasingly important. In this work, LS was introduced into ultra-high-performance concrete (UHPC) as a partial substitute for cement to explore its applicability in low-cement UHPC systems. The fresh properties of UHPC, including flowability and setting behavior, were measured, and its mechanical performance was evaluated through compressive and flexural strength tests. In addition, early-age autogenous shrinkage was monitored to clarify the effect of LS on dimensional stability. To further reveal the mechanisms associated with the macroscopic performance changes, isothermal conduction calorimetry, backscattered electron microscopy (BSE), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) were adopted. These techniques were used to characterize the hydration behavior, phase composition, hydration product evolution, and pore-structure characteristics of UHPC containing different LS contents. Results demonstrate that LS incorporation effectively reduces autogenous shrinkage and accelerates setting. An optimal LS content enhances long-term strength development; however, LS incorporation compromises early-age compressive strength and flowability. Calorimetric, thermogravimetric, and BSE analyses collectively reveal that LS retards early hydration heat release and delays initial strength gain, attributable to its dilution effect, but exhibits latent pozzolanic reactivity, consuming Ca(OH)₂ and promoting secondary C–S–H formation, thereby increasing the 28-day degree of cement hydration. Pore-structure analysis further confirms that an appropriate LS content significantly reduces total porosity, average pore diameter, and the volume fraction of pores with PD > 50 nm, leading to a more refined and compact microstructure. Integrated macroscopic and microscopic evidence identifies 20 wt.% LS as the optimal replacement level: relative to the reference mixture LS0, the 28-day compressive and flexural strengths of LS20 are increased by 10.86% and 27.93%, while flowability decreases by 13.97%, initial setting time shortens by 10.54%, and autogenous shrinkage is reduced by 57.41%. The results provide a scientific basis for the resource utilization of lithium slag in UHPC and contribute to the development of cement-reduced UHPC mixtures with improved mechanical and microstructural characteristics.