<p>Ferroelectric wurtzite <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({{\rm{Zn}}}_{1{-}{\rm{x}}}{{\rm{Mg}}}_{{\rm{x}}}{\rm{O}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mi mathvariant="normal">Zn</mi> </mrow> <mrow> <mn>1</mn> <mi mathvariant="italic">−</mi> <mi mathvariant="normal">x</mi> </mrow> </msub> <msub> <mrow> <mi mathvariant="normal">Mg</mi> </mrow> <mrow> <mi mathvariant="normal">x</mi> </mrow> </msub> <mi mathvariant="normal">O</mi> </mrow> </math></EquationSource> </InlineEquation> shows significant promise due to its ferroelectric properties, scalability, and compatibility with semiconductor platforms. We develop an integrated thermodynamic modeling framework that couples CALPHAD, first-principles calculations, and Landau-Devonshire theory to predict phase stability and ferroelectric behavior in <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({{\rm{Zn}}}_{1{-}{\rm{x}}}{{\rm{Mg}}}_{{\rm{x}}}{\rm{O}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mi mathvariant="normal">Zn</mi> </mrow> <mrow> <mn>1</mn> <mi mathvariant="italic">−</mi> <mi mathvariant="normal">x</mi> </mrow> </msub> <msub> <mrow> <mi mathvariant="normal">Mg</mi> </mrow> <mrow> <mi mathvariant="normal">x</mi> </mrow> </msub> <mi mathvariant="normal">O</mi> </mrow> </math></EquationSource> </InlineEquation>. CALPHAD quantifies the solubility limit in wurtzite and delineates the critical phase boundary for supersaturation, offering insights into phase separation relevant for synthesis and processing. First-principles calculations provide composition-dependent structural, elastic, and ferroelectric properties, enabling parameterization of Landau-Devonshire ferroelectric model for wurtzite <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\({{\rm{Zn}}}_{1{-}{\rm{x}}}{{\rm{Mg}}}_{{\rm{x}}}{\rm{O}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mi mathvariant="normal">Zn</mi> </mrow> <mrow> <mn>1</mn> <mi mathvariant="italic">−</mi> <mi mathvariant="normal">x</mi> </mrow> </msub> <msub> <mrow> <mi mathvariant="normal">Mg</mi> </mrow> <mrow> <mi mathvariant="normal">x</mi> </mrow> </msub> <mi mathvariant="normal">O</mi> </mrow> </math></EquationSource> </InlineEquation> single crystals. Extending the framework to epitaxial thin films, we show how composition and biaxial strain jointly influence phase stability and room temperature functional properties. Large biaxial tensile strain stabilizes the wurtzite phase with high Mg content in thin films, unlike the equilibrium two-phase mixture with very limited Mg solubility. Meanwhile, tensile epitaxial strain reduces polarization but enhances dielectric and piezoelectric responses by driving a polar-to-nonpolar transition within the accessible composition range. Together, these results demonstrate that both chemical modification and strain engineering are essential for enabling and tuning ferroelectricity in <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({{\rm{Zn}}}_{1{-}{\rm{x}}}{{\rm{Mg}}}_{{\rm{x}}}{\rm{O}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mi mathvariant="normal">Zn</mi> </mrow> <mrow> <mn>1</mn> <mi mathvariant="italic">−</mi> <mi mathvariant="normal">x</mi> </mrow> </msub> <msub> <mrow> <mi mathvariant="normal">Mg</mi> </mrow> <mrow> <mi mathvariant="normal">x</mi> </mrow> </msub> <mi mathvariant="normal">O</mi> </mrow> </math></EquationSource> </InlineEquation>. Our unified approach establishes a comprehensive thermodynamic framework for the predictive design of strain-tunable wurtzite ferroelectrics.</p>