The earth copious extremely active photo/electrocatalysts have been of immense interest for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) together with environment purification. However, the efficiency of photo/electrocatalysis is still low because of the less visible light absorption and fast recombination of electron-hole pairs. Herein taking the benefits of layered and electronic structural design of Ni-Mn-layered double hydroxide and layered graphitic carbon nitride. A novel Ni-Mn-LDH/g-C3N4 heterostructured photo/electrocatalyst with suited bands was in situ constructed by temperature controlled hydrothermal treatment. A turn in band gap energy within the range of Ni-Mn-LDH to g-C3N4 was noted through a series of physicochemical techniques. Consequently the optimized nanohybrid Ni-Mn-LDH/g-C3N4 (10%) used as best electrocatalyst with J(OER) = 10 mAcm(-2) @ 316 mV and J(HER) = -60 mAcm(-2) @ 147 mV). Furthermore under visible light illumination it function as outperformed photo/electrocatalyst with J(OER) = 10 mAcm(-2) @ 296 mV and J(HER) = -60 mAcm(-2) @ -126 mV in 1 M KOH with a super stability. Similarly it was used for the degradation of RhB with outperformance (>= 99%) and rate constant k = 0.313 mn(-1). Rivaling the performance of expensive catalysts such as RuO2 and Pt/C and other counterparts. The enhanced photo/electrocatalytic activity ascribed to the formation of band-matched layered by layered heterojunction-accelerated charge separation. It is predicted that our temperature controlled strategy based on earth-profuse elements with structural reliability providing an innovative and inexpensive photo/electrocatalytic system for realistic energy conversion applications.