Owing to their unique mechanical performance for both seismic and non-seismic applications, carbon nanofiber (CNF)-reinforced engineered cementitious composites (ECCs) and CNF-reinforced lightweight ECCs (LWECCs) have been received great attentions since their introduction. Due to the crucial roles of material selection and environmental conditions on the mechanical performance of cementitious composites, and thus, the structural performance during the service life of civil engineering structures, analytical studies establishing the constitutive relationships for the mechanical properties of any cementitious composite are of great importance. In this study, the constitutive relationships for the mechanical properties of CNF-reinforced ECC, as well as, hollow glass microsphere (HGM)-based LWECC and CNF-reinforced HGM-based LWECC have been developed. The proposed models include compressive strength, flexural strength, and compressive stress–strain relationships at both ambient and elevated temperatures. Furthermore, the available experimental results have been applied so that the generality and workability of the established models can be evaluated. The results indicated that to establish more reliable and accurate constitutive relationships, more researches and experimental data for CNF-reinforced ECC and LWECC with and without CNF reinforcements are essential.