Magnesium alloys are considered a promising material for the production of biodegradable implants. However, their widespread adoption is hindered by their low strength and high corrosion rate. Deformation can enhance the strength characteristics; however, due to the limited number of slip planes in magnesium alloys, choosing the deformation processing regime for them is a non-trivial task. This study presents the results of research aimed at determining the influence of processing modes by high-pressure torsion on the transformation mechanisms of the structure and mechanical characteristics of the Mg-8.6Zn-1.2Zr alloy. It is shown that at room temperature, predominantly a twinning structure is formed, with microhardness values reaching 1200 MPa. However, it is demonstrated that such a structure leads to significant embrittlement of the Mg-8.6Zn-1.2Zr alloy. Deformation at a temperature of 250 °C leads to the formation of recrystallized grains with a size of 3–4 μm. After 5 revolutions of high-pressure torsion, the microhardness is 820 MPa, and the tensile strength is 335 MPa, with an elongation of 13 %.