The mechanical reliability of membrane electrode assemblies (MEAs) in polymer electrolyte fuel cells (PEFCs) is a major concern for fuel cell vehicles. Hygrothermal cyclic conditions induce mechanical stress in MEAs and cracks form under operating conditions. This paper investigates the failure mechanism of MEAs under several mechanical and environmental conditions with the aim of designing durable PEFCs. We performed static tensile tests and low-cycle fatigue tests on MEAs. During the tensile tests, the temperature and humidity of the test chamber were controlled and surface crack formation of MEAs was observed in situ by a video microscope. Low-cycle fatigue tests were performed at ambient conditions and the number of cycles to crack formation was measured. The results reveal that the temperature and the humidity affect the mechanical properties of MEA. Observations of MEAs during tensile tests reveal that cracks form on the surface of catalyst layers immediately after the MEAs yield. These results indicate that reducing the deformation mismatch between the catalyst layer and the proton exchange membrane is important for suppressing crack formation in MEAs. The results of low-cycle fatigue tests reveal that the fatigue strength of a MEA follows the Coffin–Manson law so that fatigue design of MEAs based on the Coffin–Manson law is possible. This result is valuable for designing durable PEFCs.