Membrane mechanical durability is one of the primary factors limiting the lifetime of ionomer membranes in polymer electrolyte fuel cells. Due to the importance of membrane dimensional stability during relative humidity (RH) cycles on the magnitude of mechanical stress, the present work contributes to an in-depth understanding of the hygral swelling and shrinkage of state-of-the-art membranes and catalyst coated membranes (CCMs). Through conducting ex-situ hydration and dehydration tests, the swelling and shrinkage characteristics for each membrane were determined. An overall, progressive shrinkage in the planar dimension was observed in certain membranes suggesting the creation of residual tensile stress during in-situ operation, hence diminishing the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. Finally, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding CCM and the reinforcement offered by the catalyst layers was determined. The present findings indicate critical impact of the membrane hygral swelling and shrinkage on the in-situ mechanical stresses and the associated membrane mechanical durability.