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This study examines the effects of thermal exposure in air environment on the plastic elongation of thin sheets of Ti-15Mo-2.7Nb-3Al-0.2Si (Timetal-21S) alloy. Specimens with thicknesses of 0.12, 0.39, and 1.0 mm were exposed in air environment to temperatures ranging from 482 °C to 693 °C. Tensile tests conducted on these specimens at room temperature show a reduction of plastic elongation proportional to the thermal exposure parameters, time and temperature. Furthermore, a change in the failure mode into a quasi-brittle fracture was observed in the near-surface region. The depth of this region depends on both exposure time and temperature. The kinetics of embrittlement is studied through theoretical considerations of gas diffusion into metal. This approach shows that two distinct embrittlement mechanisms operate in this alloy. The characteristics of each of these mechanisms depend on the corresponding temperature range. At temperatures higher than 545 °C, the embrittlement activation energy is 41.2 kcal·mol−1, indicating that the embrittlement process is governed by an enhanced diffusion of oxygen into Timetal-21S. Below this transitional temperature, the embrittlement activation energy approaches zero, a characteristic of slow kinetics transformation. The effects of solid-solution hardening, precipitation-hardening mechanisms, and alloying-element partitioning on ductility exhaustion processes are analyzed and discussed.