Cementitious mixtures incorporating carbon nanofibers (CNFs) have emerged as new construction materials with enhanced mechanical and electrical performance. Due to their high strength, small size, and high aspect ratio, CNFs have the potential to bridge the cracks, limit their propagation, and fill the pores inside cementitious mixtures. Therefore, there has been a growing interest in the utilization of CNFs in the concrete industry, especially for the production of ultra-high-performance concretes (UHPCs) and self-sensing cementitious mixtures for structural health monitoring applications. Thus far, several studies have focused on the compressive strength of cementitious mixtures incorporating various nanomaterials including CNFs at the age of 28 days. The enhancing effect of CNFs on the early-age compressive strength of cementitious mixtures (i.e., up to 7 days of age), however, has not been largely investigated. Such early-age mechanical performance improvements are important to promote wider applications of CNFs in the concrete industry, for example, to compensate for the delays in the strength development of concretes made with large volumes of supplementary cementitious materials (SCMs). To fill this gap, the current paper aims to evaluate the effect of incorporating CNFs in various concentrations on the heat of hydration and compressive strength development of cement pastes in the first 7 days after production. Aqueous dispersions containing different concentrations of CNFs and a polycarboxylate ether (PCE)-based superplasticizer were prepared, subjected to ultrasound energy, and added to cement pastes at the time of mixing. The superplasticizer content in each mixture was adjusted so that all the cement pastes had a similar flow. The cement pastes were tested for isothermal calorimetry and compressive strength at the ages of 1, 3, and 7 days. Furthermore, scanning electron microscopy (SEM) was used to evaluate the dispersion of CNFs in the cement pastes.