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The 2D Janus transition‐metal dichalcogenides (TMDs) and alloyed TMDs are a widely studied emerging class of 2D materials that have been extensively used in electronic devices because of their excellent electronic, optical, and mechanical properties. The properties and behaviors of 2D‐materials‐based devices, such as the electrical breakdown caused by structural failure, are significant issues that have drawn considerable attention. In this study, the electrical behavior of polymorphic molybdenum sulfide selenide (MoSSe) devices is studied via in situ biasing experiments and recorded using transmission electron microscopy (TEM) at the atomic scale. The selenization temperature is a key factor in the phase transition of the material, which further affects the electrical and mechanical properties of MoSSe. The effects of electron‐beam irradiation and bias voltage are also discussed through a combination of experiments and theory. Quantifying the defect coverage and defect size also helps us to understand the behavior of material degradation. Furthermore, Cs‐corrected scanning TEM is utilized to identify the evolution of the morphology. The fracture morphology of the synthesized structure also varies with the application of high voltage. The cracks and defects caused by Joule heating are studied in terms of fracture type and size. Janus and alloy polymorphic monolayer molybdenum sulfide selenide (MoSSe) prepared at different selenization temperatures exhibits different electrical and mechanical properties after biasing. In the structural degradation of MoSSe, powerful in situ transmission electron microscope (TEM) and annular dark‐field scanning TEM are used to understand the individual effects of electron beams and bias.