γ-TiAl based alloys are advanced structural materials use in the automotive and aerospace industries. Their notable characteristics, including low density, high specific yield strength, and exceptional resistance to creep and oxidation, make them highly viable for being used as structural components in high-temperature applications of internal combustion engines. The novel β-solidifying γ-TiAl alloy designed in this study demonstrated excellent oxidation resistance at temperatures of 750, 800, and 850 ^oC. However, research regarding the solid-state phase transformations and microstructure control of this alloy is lacking. The study of the phase transformation behavior and microstructural evolution of alloys is crucial for developing appropriate thermal processing and heat treatment techniques for β-solidifying γ-TiAl alloys. This work introduces a novel Ti-Al-Mn-Nb alloy, with a nominal composition of Ti-43Al-1.5Mn-3Nb-0.2Si-0.2C-0.1B (atomic fraction, %). Using Pandat software for thermodynamic calculations, along with techniques such as EPMA, TEM, EBSD, and XRD, an extensive and meticulous investigation of the microstructural transformations within the range from 1440 ^oC to 1000 ^oC for this innovative alloy was undertaken. The results indicate that the as-cast microstructure of the alloy comprises a lamellar colony (α₂/γ), grain γ phase, and a small amount of β_o. The solidification pathway of the alloy can be determined as follows: liquid→liquid + β→β→β + α→α→α + γ→(α₂ + γ)→(α₂ + γ) + β_o→(α₂ + γ) + β_o + γ_g. The temperature at which the alloy exists as a single β phase (T_β ) is approximately 1420 ^oC, while the decomposition temperature of γ phase (T_γ,solv) is approximately 1280 ^oC; additionally, the eutectoid transformation temperature (T_eut) is approximately 1160 ^oC. Slightly below T_{γ, solv}, the γ precipitated from the α phase exhibits a lamellar structure. The α and γ phases consistently demonstrate a Blackburn orientation relationship: (111) _γ //(0001){}_{{\alpha }_{\mathrm{2}}} and <\mathrm{1}\overline{\mathrm{1}}\mathrm{0}> _γ //<\mathrm{11}\overline{\mathrm{2}}\mathrm{0}>{}_{{\alpha }_{\mathrm{2}}}, respectively. The secondary β_o phase precipitated from the α phase appears as a block shape and follows the Burgers orientation relationship: (110){}_{{\beta }_{\mathrm{O}}}//(0001){}_{{\alpha }_{\mathrm{2}}} and <111>{}_{{\beta }_{\mathrm{O}}}//<\mathrm{11}\overline{\mathrm{2}}\mathrm{0}>{}_{{\alpha }_{\mathrm{2}}}. The Vickers hardness of the quenched microstructure of the novel alloy ranges between 385 and 512 HV. With an increase in the quenching temperature, there is an observable enhancement in the microhardness of the quenched microstructure. The martensite microstructure formed after quenching in the β single-phase area contributes to the hardness of 512 HV. This novel alloy encompasses the β and α single-phase areas; thereby holding significant implications for the development of novel, highly deformable, and high-temperature-resistant β-solidifying γ-TiAl alloys characterized with fully lamellar structures.