Anisotropic assemblies of magnetic nanoparticles (NPs) are highly powerful for designing micro-actuators in various applications, from robotics to biomedicine. Despite great potential, the challenge remains of how to controllably fabricate them at the micro scale due to two limitations: i) the lack of high-remanence nanoscale magnetic materials, which would dictate the resolution and displacement of the actuator; and ii) the lack of robust method for the large-scale synthesis of such materials. In this work, iron cobalt (Fe-Co) magnetic nanochains are prepared with a novel strategy, where the nanochains are formed from dipole-dipole alignment of individual Fe-Co NPs in the absence of an external magnetic field. As opposed to the widely studied field-guided assembly, our spontaneous assembly reported here enables high-throughput fabrication of anisotropic Fe-Co chains. Experiments and simulations show that dipolar magnetism plays a key role in the resulting structures of the Fe-Co chains, whose morphology, width, and length can be tuned by varying the amounts of precursors and fluid rotating speed during the synthesis. The randomly-oriented Fe-Co chains show a remanent magnetization of 28 emu/g, increasing by a factor of 2 compared to the Fe-Co NPs with equal side lengths. Furthermore, this higher remanence in the nanochains is experimentally shown to be advantageous in the development of magnetic soft actuators with a much larger mechanical deformation response to an externally applied field. To demonstrate this point, anisotropic magnetic elastomer composites were fabricated and their mechanical deformation behavior tested in both bending and twisting actuation modes. Both cases showed, very large, visible deformations in low magnetic fields (< 200 Oe). Thus, by developing high-remanence magnetic chains with enhanced yields, composite materials can be made to maximize their effectiveness as micro-actuators or as other device technologies.