A mechanical route using a grinding apparatus such as a planetary ball mill is a simple and scalable method to produce powder materials. However, the control of the particle shapes is difficult. In this paper, we report a wet mechanical process in water to synthesize NH4MnPO4·H2O (AmMnP) with various shapes (plates, flakes, rods, and nanoparticles). This process involves planetary ball milling of inexpensive raw materials (NH4H2PO4 and MnCO3) at room temperature. Morphology-controlled AmMnP particles can be obtained by only adjusting the milling conditions such as milling time, ball size, and centrifugal acceleration. Furthermore, the conversion of AmMnP into LiMnPO4 with two different approaches (solid-state and hydrothermal reactions) has been investigated to evaluate its future applicability as a cathode for lithium-ion batteries. As a particle synthesis with a unique morphology can be attained based on a dissolution-precipitation mechanism in a solution via a suitable combination of raw materials, the study results will promote wet mechanical processes to be widely used as classic but advanced particle synthesis method.