The fast degradation rate of magnesium alloys in biological environments limits its application/use in biodegradable metallic implants. The desirable corrosion rates and mechanical properties in magnesium alloys for orthopaedic applications could be achieved through the microstructural changes caused by thermomechanical processing. In this work, the effect of hot rolling and thermal treatments to the peak-aged and the over-aged conditions on the microstructure, hardness and biocorrosion resistance of a cast Mg-3wt.% Zn-0.4wt.% Ca (ZX30) alloy was studied. Microstructural characterisation was performed by optical microscopy, scanning electron microscopy and electron backscatter diffraction, while the corrosion behaviour in biological environments was evaluated by immersion and electrochemical impedance tests in Hank’s solution. The results show the beneficial effect of combining plastic deformation and heat treatment on the mechanical and biocorrosion behaviour of the ZX30 alloy. This was discussed in terms of grain structure, crystal orientation, dislocation density and precipitate distribution.