The effect of a polycarboxylate-based high-range water reducer (HRWR) on incorporation of carbon nanofibers (CNFs) in cement composites was evaluated. Use of the HRWR proved successful in disaggregating the CNFs in solution and improved the dispersion of CNFs in the cement paste at the individual fiber level. However, an inhomogeneous distribution of the fibers within the composites was observed. Addition of 0.2% CNFs per weight of cement resulted in increased splitting tensile strength of 22% in portland cement composites and 26% in composites that also contained silica fume. Migration of the CNFs in the bleed water was observed depending on the water–binder ratio used, creating a porous layer of agglomerated CNFs intermixed with cement paste at the upper surface of the composite.