We have investigated room-temperature stretch formability, age-hardening behavior, tensile properties, and microstructures of Mg–6Zn-0.2Ca-0.3Ce and Mg–6Zn-0.2Ca-0.4Y (mass%) alloy sheets. In hot-rolled and solution-treated sheets, (0001) planes of most grains split into both rolling and transverse directions, resulting in large Index Erichsen values of 7.6 ± 0.5 mm and 7.7 ± 0.2 mm, respectively. The Y-containing sheet shows lower age-hardenability than the Ce-containing alloy sheet;​ however, the Y addition led to a significant grain refinement, suppressing the ductility loss after peak aging. This results in the improved balance of strength and ductility in the Mg–6Zn-0.2Ca-0.4Y alloy sheet, and subsequent peak-aging led to high 0.2% proof stresses of 228 ± 1 MPa and large elongation to failure of 19 ± 1% along the rolling direction. Quasi-in-situ EBSD analysis of texture development during a solution treatment indicated that twinning-mediated recrystallization mainly contributes to the texture weakening in the high-alloyed Mg–Zn–Ca based alloy sheets.