Thermochromism and gradual spin crossover (SCO) were observed in the [MnIII(5-MeO-sal-N-1-5-8-12)]Cl complex. The transition temperature (T1/2) was determined to be 281 K by the drastic changes in the unit cell. This T1/2 value is the highest observed for this type of system using the 5-MeO-H2sal-N-(1-5-8-12) ligand. The gradual transition was monitored by single crystal X-ray diffraction at four temperatures (100 K, 290 K, 350 K and 400 K), and the octahedral deviation parameters were obtained to classify the spin state. Moreover, the unit cell parameters were determined at 12 different temperatures from 90 K to 400 K, and significant changes were observed in the unit cell from 250 K to 310 K, corresponding to the low-spin (LS) state to high-spin (HS) state transition or vice versa. Thermochromism in the solid state was observed at cryogenic temperatures, starting at 250 K on a single-crystal sample, representing clear evidence of the electronic transition. This work opens further opportunities to design and synthesize new SCO profiles for MnIII complexes using Schiff base derivatives.