The (CH3Mg)-C-12, (CH3Mg)-C-13-Mg-25, (CH3Mg)-C-12-Mg-25, (CD3Mg)-C-12-Mg-25, (CH3Mg)-C-13-Mg-25 and (CD3Mg)-C-13-Mg-25 radicals have been isolated in an inert neon matrix at 4.3 K and their electronic structure probed, for the first time, using matrix isolation electron spin resonance (MI-ESR) spectroscopy. These radicals were formed from the reaction of laser-ablated magnesium metal and an appropriately labeled derivative of acetone or methyl iodide. The spin Hamiltonian parameters, g(perpendicular to) = 1.9999(4), A(perpendicular to)(Mg-25) = -184(1) MHz, A(perpendicular to)(C-13) = 128(2) MHz and A(perpendicular to)(H) = 7(1) MHz were determined from an exact diagonalization analysis of the experimental spectra and estimates were derived for A(parallel to)(Mg-25) = -197(10) MHz and A(parallel to)(C-13) = 180(20) MHz assuming g(parallel to) = 2.0023. A model for the bonding in the CH3Mg radical is derived using this hyperfine data. Comparisons are made between the CH3Mg radical and other related magnesium and monomethylmetal radicals, MgH, MgOH, CH3Cd, CH3Zn. and CH3Ba. Theoretical nuclear hyperfine coupling constants for the CH3Mg radical were evaluated using Hartree-Fock single and double excitation configuration interaction (HFSDCI), multireference single and double excitation configuration interaction (MRSDCI) and density functional theory (DFT) ab initio calculations. While these theoretical methods yielded values for A(dip)(Mg-25) and A(dip)(C-13) in agreement with the experimental values, the calculated A(iso)(Mg-25) value was low by 4%(HFSDCI) and 15% (MRSDCI). Whereas the calculated A(iso)(C-13) values were low by 50% (HFSDCI) and 32% (MRSDCI). Unrestricted DFT calculations using the B3PW91 and B3LYP functionals yielded values of A(iso)(Mg-25) low by approximately 15% for both functionals and values of A(iso)(C-13) in agreement with experiment for UB3LYP and low by 10% for UB3PW91. The discrepancy between the calculated and experimental values of A(iso)(C-13) for the CI results is attributed to the limited reference space resulting in an overestimation of the ionic character in the bonding of the CH3Mg radical.