• Incorporating nanotwins (3–30 nm width) into Cu/CNTs composites via cryogenic rolling and optimized powder particle size significantly enhances strength–ductility synergy.
• Nanotwin strengthening contributes 19.9% of the total strength increment, alongside grain refinement and dislocation strengthening.
• The improved ductility is attributed to enhanced dislocation accommodation and reduced dislocation proliferation due to nanotwins.
• This work demonstrates the feasibility of combining nanotwins with ex-situ reinforcements to overcome the strength–ductility trade-off in metal matrix composites.