In this paper, ultrathin (about 2.5 nm) manganite (MnOOH) nanowires were synthesized by a simple wet chemical method at ambient temperature. Temperature-dependent phase transition patterns of the prepared nanowires under different atmospheres were systematically studied with thermogravimetric and differential thermal analysis (TG/DTA). Based on the critical temperatures demonstrated by the phase transition pattern, manganese oxides, such as Mn2O3 and Mn3O4, have been obtained conveniently through calcination. Scanning electron microscope (SEM), transmission electron microscope (TEM) and x-ray diffraction results confirm that the annealed nanowires can both retain their original 1D morphology and show good crystallinity. The electrocatalytic performance of the as-prepared manganese oxides was also explored, which showed good response to the reduction of H2O2 in alkaline media. Among the three manganese oxides, the MnOOH nanowires exhibited the most prominent electrocatalytic efficiency and stability, which provides a good candidate for modern industrial catalysis applications.