The (HgCH3)-C-12, (HgCH3)-C-13, (HgCD3)-C-12,(HgCH3)-Hg-199-C-12, (HgCH3)-Hg-201-C-12, (HgCH3)-Hg-201-C-13, (HgCD3)-Hg-199-C-12, and (HgCD3)-Hg-201-C-12 radicals have all been formed in a microwave discharge and isolated in an inert neon matrix. Their electronic structure was established for the first time using electron spin resonance (ESR) spectroscopy. The following magnetic parameters were determined from the experimental spectra, g(perpendicular to) = 1.84525(20), A(perpendicular to)(Hg-199) = 4337(3) MHz, A(perpendicular to)(Hg-201) = 1604(1) MHz, A(perpendicular to)(C-13) = 146(2) MHz, A(perpendicular to)(H) = 31(1) MHz, and A(perpendicular to)(D) = 5(1) MHz. Estimates were made for the following magnetic parameters, g(\\) = 1.990(5), A(\\)(Hg-199) = 5658(10) MHz, A(\\)(Hg-201) = 2094(10) MHz, and A(\\)(C-13) = 270(25) MHz. The free atom comparison method (FACM) was used to determine the unpaired electron spin density distribution for the radical and to develop a bonding model. The HgCH3 radical results are compared with our previous results for the ZnCH3 and CdCH3 radicals as well as other mercury-containing radicals such as HgH, HgCN, and HgF.