An increase in interfacial properties between the matrix, a polyurethane cellular foam, and the reinforcement, a short carbon fibre, led to improved mechanical properties of a light-weight composite. The carbon fibre surface modification was designed with two aims: to impart magnetic properties so the discontinuous fibres could be aligned on-demand during the manufacturing process using a weak magnetic field, and to promote interfacial adhesion between the matrix and the reinforcement. After surface treatment, functionalising and coating with magnetite nanoparticles created and deposited in situ via electrodeposition prior to their deployment, the fibres were susceptible to magnetic manipulation and orientation within the reacting foam. The fibre coating contributed to interfacial compatibilization between the matrix and the reinforcement. Comparing the results between unreinforced, reinforced with untreated fibre, and reinforced with magnetised fibre, the results show that: foam reinforced with a low %vol content, i.e., from 0.1%vol to 0.4%vol, of any of the fibres improved specific strength, stiffness and toughness in tension relative to the unreinforced cellular polymeric matrix without densification, modification of cell size or compromising their lightweight properties. The magnetised fibre-containing composites showed significantly improved mechanical properties overall in tension, when compared to the untreated fibres, due to their enhanced interfacial adhesion and their alignment in the matrix. Results in compression yielded improvement only in compressive strength, with other properties being similar to the unreinforced matrices. No significant differences were observed between the magnetised (aligned fibres) and the untreated (randomly distributed) configurations in compression.