We propose the phase transformation of magnesium hydroxide (brucite) to magnesium-aluminum hydroxide (layered double hydroxide: LDH) utilizing solid state brucite and aqueous aluminum(III) as precursors in order to obtain highly crystalline and large-sized LDH. Under a hydrothermal reaction at 150 °C, the brucite was partially dissolved, and aqueous aluminum precipitated in the form of boehmite within 1.5 h. Then, the precipitated aluminum migrated into the brucite framework to transform the crystal phase of brucite to LDH within 2.3 h of reaction. Time-dependent X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy analyses showed the time-dependent evolution of LDH from brucite. The transformed LDH exhibited crystal growth along the ab-plane direction first followed by crystal growth along the c-axis. Quantitative analysis utilizing inductively coupled plasma-optical emission spectroscopy for both the solid part and supernatant confirmed that the phase transformation was mediated by both dissolution-reprecipitation and isomorphous substitution in the solid state. The solid-state magic angle spin nuclear magnetic resonance spectroscopy for 27Al indicated that the crystal growth of phase-transformed LDH was accompanied by local ordering around Al(III) in LDH.