Herein, we first show that layered double hydroxide derived ultrathin 2D Ni-V mixed metal oxide (MMO) nanosheets with thickness of sub-10 nm exhibit an robust peroxidase (POx) mimic activity, accelerating H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to produce a color reaction. The effect of varying the Ni/V molar ratio and heating temperature on the POx mimic activity and physicochemical properties of Ni-V MMOs has also been investigated in detail. Ni-V MMO is composed of NiO and Ni3V2O8 and under optimized conditions, Ni3V2O8 nanoparticles with particle size of sub-5 nm are well dispersed on the 2D NiO nanosheet matrix. Ni-V MMO with a Ni/V molar ratio of 3:1 calcinated at 400 degrees C shows best POx mimic activity. The catalytic mechanism was analyzed by electron spin resonance spectroscopy, which suggested that the enzyme mimic activities of Ni-V MMO originated from the decomposition of H2O2 to generate reactive (OH)-O-center dot radicals. Kinetic analysis indicates typical Michaelis-Menten catalytic behavior. The findings were used to design a colorimetric assay for H2O2, best measured at 652 nm. The method has a linear response in the 10-100 mM H2O2 concentration range, with a 5.7 mM detection limit. Benefitting from the high dispersibility and high specific surface area of the ultrathin 2D nanosheets, the method is well reproducible. It's also easily performed and highly specific. The current work can provide new strategies to construct novel LDH derived nanoenzymes for biosensor, biotechnology, and biomedicine.