Electronic structure methods and nonperturbative resonance theory are applied to study the radiative and radiationless decay mechanisms of the MgBr (A (2) Pi(Omega)) vibrational levels. The X (2) Sigma(+) and 1,2 (2) Pi(Omega) adiabatic electronic states are characterized using ab initio state-averaged multiconfigurational self-consistent field/second order configuration interaction wave functions. Interstate derivative couplings between the (2) Pi states have been calculated and used to construct a rigorous diabatic basis. The nonrelativistic potential energy curves are modified in the first order of degenerate perturbation theory to take account of the spin-orbit interactions treated within Breit-Pauli approximation. All vibrational levels in the A (2) Pi(Omega) manifold are resonances predissociated by the repulsive 2 (2) Pi state. A recently developed computational approach [S. Han and D. R. Yarkony, Mel. Phys. 88, 53 (1996)] based on a Feshbach formalism is employed to determine energies, linewidths, and radiative and radiationless decay rates in a coupled diabatic states basis within a Hund’s case (a) approximation. Large nonadiabatic interactions cause significant energy shifts in the resonances levels. It is shown that a pronounced Omega-dependence in the radiationless decay rates results from the large fine structure splitting in the 2 (2) Pi(Omega) diabatic state which corresponds to Mg(S-1)Br(P-2). Comparisons with absorption and fluorescence spectra reveal important insights into A (2) Pi(Omega) state decay. The spectroscopic constants of the A (2) Pi(Omega), Omega=3/2 and 1/2 states and the A (2) Pi(3/2) state predissociation are well described in a Hund’s case (a) approximation. However it is found that the A (2) Pi(Omega) state predissociation is significantly underestimated in this limit. Rather the A (2) Pi(1/2) state is indirectly predissociated by the 2 (2) Pi(3/2) State through rotational coupling to the A (2) Pi(3/2) state.