Multicomponent organic crystals based on natural metabolites or pharmaceutical active ingredients play an important role in solid state chemistry, agriculture, and pharmaceutical industry. This study reveals the synthesis of a new luminescent alkanolammonium salt of 4-nitrobenzoic acid (MMEA4NB, methylethanolammonium 4-nitrobenzoate), together with the detailed investigation of its structural motifs, thermal stability, decomposition kinetics, and validation of its plant growth regulatory activity. The new compound was obtained at room temperature in excellent yield and shorter reaction time. Thermal stability and decomposition were studied in a dynamic air atmosphere by thermogravimetry—differential thermal analysis—derivative thermogravimetric techniques. The kinetics and mechanism of the thermal decomposition reaction were evaluated under non-isothermal conditions using three different isoconversional methods Flynn–Wall and Ozawa, Friedman, and modified nonparametric kinetics method in order to obtain the activation energy. Proton transfer and hydrogen bonding in the organic salt were elucidated and confirmed by infrared spectroscopy and single-crystal X-ray diffraction. The crystal structure revealed cation–anion heterosynthons and cation–cation homosynthon. The relation between the protonation state of the components, intermolecular interactions, and thermal stability of MMEA4NB is discussed. This study demonstrates the importance of MMEA4NB in the development of new specific luminescent plant growth regulators, possible viewers of localization and distribution in plants.