\textbf{Background}: Superallowed decays of isospin nuclides can be used to test theoretical isospin symmetry breaking corrections applied to extract the CKM matrix element from decays by measuring precise values and also to search for scalar currents using the angular correlation. Key ingredients include the value and branching of the superallowed transition and the half life of the parent. \textbf{Purpose}: To determine a precise experimental value for the superallowed decay of Mg and the intensity of Mg -delayed rays through the isobaric analog state in Na. \textbf{Method}: A beam of Mg was produced using the in-flight method and implanted into a plastic scintillator surrounded by an array of high-purity germanium detectors used to detect -delayed rays. The high-resolution -ray spectrum was analyzed to measure the -ray energies and intensities. \textbf{Results}: The intensity of Mg -delayed rays through the isobaric analog state in Na was measured to be $(1.60 \pm 0.04_{\textrm{stat}} \pm 0.15_{\textrm{syst}} \pm 0.15_{\textrm{theo}}) \times 10^{-4}$, where the uncertainties are statistical, systematic, and theoretical, respectively. The value for the superallowed transition was determined to be keV based on the measured excitation energy of keV and literature values for the ground-state masses of Na and Mg. \textbf{Conclusions}: The -delayed -decay branch and value are now sufficiently precise to match or exceed the sensitivity required for current low-energy tests of the standard model.