The isotropic average, J(HgC)(iso), and the anisotropy, Delta J(HgC), of the Hg-199-C-13 spin-spin coupling tensor in methylmercury halides, CH3HgX (X = Cl, Br, I), were determined for the first time by utilizing the NMR spectra of these molecules dissolved in liquid crystals. Furthermore, density functional calculations were performed using the zeroth-order regular approximation, including also dimethylmercury. The temperature-dependence of the J(HgC)(iso) couplings in the isotropic phase was studied in each case in order to extrapolate their values into the liquid crystal state. Good agreement is found between the experimental and the calculated Delta J(HgC) values as long as solvent effects are considered in the computations. Most of the magnitude of Delta J can be attributed to the spin mechanism of J-coupling, with additional sizable spin-orbital cross terms due to electronic spin-orbit coupling.