Perovskite materials have attracted much attention in heterogeneous catalysis. Here, we report a perovskite type material LaNiO3-delta (LNO) for degradation of methyl orange (MO) azo dye in aqueous solutions under dark ambient conditions (room temperature, atmospheric pressure) without additional lights or chemical stimulants. The high degradation percentage of 5 ppm MO with 1.5 g/L LNO was 94.3% after 4 h with a stirring speed of 500 rpm. Reactions under nitrogen, oxygen and carbon dioxide conditions were performed with efficiencies of 19.6%, 7.1%, and 96.8% respectively after 6 h to understand the mechanism. MO was shown to degrade under dark ambient condition via main intermediates, sulfanilic acid anion and N,N-dimethyl-p-phenylenediamine (DPD), by electrospray ionization mass spectrometry (ESI/MS) and high performance liquid chromatography (HPLC). Degradation of MO under such a mild condition is due to two synergic effects proposed by means of XRD, FTIR, TGA, SEM, and XPS. Nickel is oxidized during MO degradation; lanthanum carbonate (La-analog calkinsite) is formed on the LNO surface due to the aqueous solution environment and speeds up azo bond cleavage. This work unravels the mechanism behind MO degradation by LNO under dark ambient conditions for the first time. It is a fundamental information on perovskite for dye degradation, especially for lanthanum series of perovskite. Excellent perovskite materials should be tailorable for water remediation applications considering the large variety of perovskites in terms of constituents and composition.