Based on the oxygen ion conductor La2Mo2O9, a series of K, Fe, and Mn doped samples La1.95K0.05Mo2−xTxO9−δ (T=Fe,Mn; x=0,0.025,0.05,0.1) were prepared with conventional solid-state reaction method. The effects of Fe and Mn doping on the cationic and anionic ions diffusion, phase transition, and electrical conductivity were studied. The mechanical spectroscopy (or internal friction) technique was exploited to investigate the microscopic transport mechanism and to deduce the dynamical relaxation parameters. Three internal friction peaks (PL, PH, and PM) were observed in the La1.95K0.05Mo2−xTxO9−δ samples, among which PL and PM are of relaxational type while PH is associated with a kind of phase transition. The results revealed that peak PL is originated from the short-distance jumps of oxygen vacancies, while peak PM is associated with the short-distance diffusion of Mn (Fe) ions and peak PH with the phase transition from the static disordered state to the dynamic disordered state of oxygen ion distribution. Based on the results of conductivity measurements, it can be concluded that the high-temperature conductivity of La2Mo2O9 can be improved obviously by the Fe or Mn doping at Mo site and the low-temperature conductivity can be increased by the simultaneous K doping at La site.