Industrial waste valorization is one of the major goals of sustainable development and drives research efforts to provide sustainable disposal strategies to protect the environment. This paper studies the effect of waste rockwool (RW) addition on the mechanical, thermal, and fire resistance performance of normal and lightweight (LW) mortars made with limestone-calcined clay cement (LC3). LC3 binder was prepared by substituting 60% of ordinary Portland cement (OPC) with limestone (LS) powder and metakaolin (MK) at ratio of LS-to-MK of 1:2 (wt%). Waste RW was incorporated in different ratios ranging from 1% to 5% (by weight of binder) into two types of LC3-based mortars made with natural sand (NS) and expanded polystyrene (EPS). The binder-to-aggregate ratio of 1:3 by volume. After 28 days of curing, properties including compressive strength, density, and thermal conductivity were investigated. The residual compressive strength after exposure to elevated temperature performance for up to an hour was determined to assess the fire endurance of the prepared LC3 mortars blended with RW. The incorporation of waste RW has improved the physical packing and led to enhanced compressive strengths of normal and lightweight LC3 mortars. The greatest enhancements of about 24% and 110% were obtained for the compressive strength of normal and lightweight LC3 mortars, respectively. Even at the highest dosage of RW, the results yielded low thermal conductivity of 0.37 W/m.K. After 1 h of fire exposure, residual strength of about 59% and 83% was maintained by the normal and lightweight LC3 mortars, respectively, upon incorporating 15wt.% waste RW.