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The change in electrical characteristics of a-Si:H thin-film transistors (TFTs) was determined in the presence of electrical gate bias stress, gamma radiation, and both simultaneously, simulating the harsh environment of space. Multiple TFTs were tested under each condition, and the current-voltage characteristics were measured. The results show the gate bias stress increasing the threshold voltage (VT) with power law time dependence while the gamma irradiation decreases threshold voltage for all working transistors. When both the irradiation and gate bias stress were applied simultaneously, the VT initially increased with electrical stress and then decreased as the gamma radiation dominated. Changes in effective mobility were also extracted and detailed analysis of the current-voltage characteristics indicated that the gamma radiation creates interface traps and electron-hole pairs whereas the gate stress produces defect states in the amorphous silicon.