<p>Ultra-high strength Mg-12.2Gd-2.2Y-1.2Zn-0.5Mn(wt.%) alloys were developed through hot extrusion, cold-rolling, and aging treatments. This study investigates the effects of rolling reductions (0, 3.0, 5.6, 9.6%) on the microstructure and mechanical properties of the Mg-Gd-Y-Zn-Mn alloys. Results revealed the presence of a bimodal-grained structure in the deformed alloys. Cold-rolling promoted grain refinement and increased the proportion of refined grain during subsequent aging. The ultimate tensile strength of the peak-aged alloys initially increased and then decreased with increasing rolling reductions. Following a 5.6% rolling reduction and aging treatment, the extruded Mg alloy exhibited excellent mechanical properties, achieving an ultra-high ultimate tensile strength of 571&#xa0;MPa, a tensile yield strength of 493&#xa0;MPa and an elongation to failure of 3.7%. In contrast to the extrusion-aged alloy, the enhancements in tensile yield strength and ultimate tensile strength of the E-5.6%CR-aged alloy were attributed to the combined effects of fine <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\beta^{\prime }\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>β</mi> <mo>′</mo> </msup> </math></EquationSource> </InlineEquation> precipitates, a high fraction of fine grains and high-density dislocations within the recrystallized and unrecrystallized grains. Although higher yield strengths can be obtained with a 9.6% rolling reduction, excessive rolling reduction is detrimental to tensile strength due to reduced elongation.</p>