New high-resolution visible Fourier transform emission spectra of the A 2 coproduct ->-1 x Sigma, and B' 2E+ > X 2r systems of 24MgD and of the B' 2r x 25,26. 25,26 systems of mgD and MgH have been combined with earlier results for 24MgH in a multi-isotopologue direct-potential-fit analysis to yield improved analytic potential energy and Born-Oppenheimer breakdown functions for the ground X 2r state of MgH. Vibrational levels of the ground state of 24MgD were observed up to v" = 15, which is bound by only 30.6 +/- 0.10 cm(-1). Including deuteride and minor magnesium isotopologue data allowed us also to determine the adiabatic Born-Openheimer breakdown effects in this molecule. The fitting procedure used the recently developed Morse/Long-Range (MLR) potential energy function, whose asymptotic behavior incorporates the correct inverse-power form. A spin-splitting radial correction function to take account of the 2E spin-rotation interaction was also determined. Our refined value for the ground-state dissociation energy of the dominant isotopologue (24MgH) is De = 11 104.25 +/- 0.8 cm(-1) I, in which the uncertainty also accounts for the model dependence of the fitted De values for a range of physically acceptable fits. We were also able to determine the marked difference in the well depths of 24MgH and (24)/MgD (with the deuteride potential curve being 7.58 +/- 0.30 cm(-1) deeper than that of the hydride) as well as smaller well-depth differences for the minor Mg-25'26 isotopologues. This analytic potential function also predicts that the highest bound level of (MgD)-Mg-24 is v" = 16 and that it is bound by only 2.73 +/- 0.10 cm(-1.)