The leaf extract of Azadirachta indica (Neem) plant was utilized as reducing agent for the green synthesis of Mn3O4 nanoparticles (NPs). The crystalline analysis demonstrated the typical tetragonal hausmannite crystal structure of Mn3O4, which confirmed the formation of Mn3O4 NPs without the existence of other oxides. Green synthesized Mn3O4 NPs were applied for the catalytic thermal decomposition of ammonium perchlorate (AP) and as working electrode for fabricating the chemical sensor. The excellent catalytic effect for the thermal decomposition of AP was observed by decreasing the decomposition temperature by 175°C with single decomposing step. The fabricated chemical sensor based on green synthesized Mn3O4 NPs displayed high, reliable and reproducible sensitivity of ∼569.2μAmM(-1)cm(-2) with reasonable limit of detection (LOD) of ∼22.1μM and the response time of ∼10s toward the detection of 2-butanone chemical. A relatively good linearity in the ranging from ∼20 to 160μM was detected for Mn3O4 NPs electrode based 2-butanone chemical sensor.