The extrinsic size of both crystalline alloys and amorphous metallic alloys strongly affects their mechanical properties at the submicron scale or nanometre scale. For example, Zr-based metallic glass nanopillars exhibit ceramic-like strengths (2.25 GPa) and metal-like ductility (25%) simultaneously when the pillar dimension is reduced to <100 nm. Here, we report a new compositional design approach to create tough metallic glass composites consisting of micrometre-scale dendrites and nanometre-scale amorphous matrices that exhibit high strength and ductility in the normally brittle MgZnCa metallic glass system. When the thickness of the amorphous matrix is reduced to the nanometre scale, a low density (ρ ≈ 1.99 g cm−3) Mg91.5Zn7.5Ca1 alloy exhibits room temperature tensile ductility exceeding 15.6%, a yield strength of 215 MPa and a fracture strength of 478 MPa. Transmission electron microscopy analysis demonstrates that the alloy consists of micrometre-scale α-Mg solid solution dendrites and nanometre-scale amorphous matrix (80–530 nm in thickness). The homogeneous deformation of nanometre scale amorphous matrices is believed to be responsible for the high toughness and excellent work-hardening behaviour.