The phase transition characteristics, dielectric relaxation behavior, and ferroelectric hysteresis in undoped and Mn-doped 0.24Pb(In1/2Nb1/2)O3-0.42Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 (24PIN-42PMN-34PT) ternary ceramics have been investigated. The degree of diffuse phase transition (DPT) is enhanced by the addition of manganese due to the disorder-enhanced structure. The temperature dependence of the dielectric permittivity can be well described by the Lorentz-type quadratic relationship at the temperature range of Tm-600K and Curie-Weiss law at T > 630K, respectively. In addition, the frequency dispersion of dielectric permittivity is greatly suppressed and hysteresis loops are pinched at room temperature, which is attributed to the internal bias field created by the defect dipoles after introducing Mn ion.