  Beta-delayed proton and gamma emission in the decay of $^{21}$Mg has been
measured at ISOLDE, CERN with the ISOLDE Decay Station (IDS) set-up. The
existing decay scheme is updated, in particular what concerns proton
transitions to excited states in $^{20}$Ne. Signatures of interference in
several parts of the spectrum are used to settle spin and parity assignments to
highly excited states in $^{21}$Na. The previously reported $\beta$p$\alpha$
branch is confirmed. A half-life of 120.5(4) ms is extracted for $^{21}$Mg. The
revised decay scheme is employed to test mirror symmetry in the decay and to
extract the beta strength distribution of $^{21}$Mg that is compared with
theory.
