The spin-crossover properties of the strongly cooperative compound [Fe(PM-PEA)2(NCS)2] (with PM-PEA = N-(2′-pyridylmethylene)-4-(phenylethynyl)aniline) have been investigated under external in situ pressure, external ex situ pressure and internal pressure. In situ single-crystal X-ray diffraction investigations under pressure indicate a Spin-Crossover (SCO) at about 400 MPa and room temperature. Interestingly, application of ex situ pressure induces the irreversible enlargement of the hysteresis width, almost independently from the pressure value. Elsewhere, the internal pressure effects are examined through the magnetic and photomagnetic investigations on powders of the solid-solutions based on the Mn ion, [FexMn1−x(PM-PEA)2(NCS)2]. Growing the Mn ratio increases the internal pressure, allowing to control the hysteresis width and the paramagnetic residue but also to enhance the efficiency of the photo-induced SCO. The comparison of the quenching and light-induced behaviors reveals a complex phase-diagram governed by internal pressure, temperature and light.