The effective g-factor, obtained from the spin contribution to the Knight shift, is calculated from first principles for ternary semiconductors and applied to Pb_1-xSn_xTe. A six-level k. pi theory, within the framework of the effective-mass representation, is used to calculate the effective g-factor as a function of carrier density, tin concentration and temperature. The temperature and concentration dependence of the energy gap is used via the virtual-crystal approximation. The agreement with the available experiments is good, and the trends and results obtained are in overall conformity with that found in the experiments.