Lithium-sulfur (Li-S) batteries have exhibited tremendous potential among the various secondary batteries benefitting from their large energy density, low expense, and enhanced security. However, the commercial use for Li-S batteries is immensely limited by the insulation of S, noticeable volume expansion from S to Li2S2/Li2S, and the undesired shuttle effect of lithium polysulfides (LiPs). Herein, a composite sulfur host has been prepared by in situ incorporations of cobalt nanoparticles (NPs) into nitrogen-doped mesoporous carbon spheres (Co/N-PCSs) through the composite micelle assembly strategy. The resultant functional Co/N-PCSs not only possess uniform spherical morphology with large open mesopores, high surface area, and pore volume but also have small Co NPs homogeneously inlaid into the pore walls of carbon frameworks. Both the experimental and theoretical calculation results demonstrate that the formed cobalt NPs can efficiently accelerate the lithium-ion diffusion reaction and greatly entrap the soluble intermediate LiPs. Benefiting from the well-designed structure, the Co/N-PCSs@S cathode with a S loading of 73.82 wt % delivers superior electrochemical performance, including long cycling stability (60% for the residual capacity at 1 A g-1 within 300 cycles) and excellent rate performance (∼512 mAh g-1 at 6 A g-1). This design strategy of implanting metal NPs in mesoporous carbon can be inspiring in energy storage applications.