Nonstoichiometric ordering of magnesium and niobium cations in undoped and lanthanum-doped lead magnesium niobate solid solutions has been investigated by means of high-resolution transmission electron microscopy and computer image simulation. High-resolution lattice images were obtained using different objective apertures for various microscope imaging conditions. Computer image simulations were performed for a wide range of sample thicknesses, defocusing conditions, local Mg/Nb ratios, and long-range order parameters. The simulated images demonstrated that the lattice images of the ordered region are predominantly dependent on the long-range order parameter. By a comparison of the experimental and simulated images, a range of values of the long-range order parameter 0.2 less-than-or-equal-to S < 0.8 was obtained for the ordered regions. It was also shown that the ordered structure has a (NH4)3FeF6 structure, which consists of alternating magnesium- and niobium-preferred sublattices along the [111] directions.