As the prototype of relaxor ferroelectrics, Pb(Mg_1/3Nb_2/3)O₃ (PMN) has been extensively investigated for several decades. There are two types of microscopic features in PMN: cation order and electric dipole order. The development of long range cation order in PMN has proved to be challenging and the relationship between these two types of order is still open for debate. In this work, different kinds of cations and/or defects were introduced into Pb(Mg_1/3Nb_2/3)O₃ to develop long range cation order. The impact of the enhanced cation order on the dielectric properties of Pb(Mg_1/3Nb_2/3)O₃-based ceramics was systematically investigated. Ceramic samples were prepared via the columbite method to ensure phase purity. High temperature thermal treatments were used to manipulate the degree of cation order of the material. Dielectric and ferroelectric properties, including relative permittivity, thermal depolarization current and polarization vs. electric field hysteresis, were measured. It was shown that the cation order in these relaxor ceramics was significantly enhanced. However, different dielectric and ferroelectric properties were observed in these perovskite materials. The difference was interpreted in terms of size/charge mismatch of two B-site sublattices and size/charge difference between them. The electric field--temperature phase diagrams of PMN-based ceramics and PMN single crystals were also constructed based on electric field-induced phase transitions.