This work proposes a theoretical description of ion exchange in glass involving mechano–electrochemical coupling, distinct from the previous models. The generalized Maxwell viscoelastic constitutive relation and large‐deformation formulae are employed, which is indispensable for thin glass panels widely used in consumer electronics. With the theory, two‐dimensional numerical simulations based on axisymmetric and plan‐strain models are conducted. The numerical results signify lateral deformation in a double‐side ion‐exchanged glass and bending caused by single‐side or electric‐field‐assisted ion exchange. With the decrease in glass thickness, the increase in time, or applied electric potential, these deformations become more significant, leading to the relief of in‐plane stresses in the ion‐exchanged layer and the increase in stresses in other parts. This could lead to a remarkable decrease in fracture strength or cracking of an ultrathin glass panel. As the stress and diffusion are coupled, the decrease in stress in the ion‐exchanged layer leads to the increase in the depth of layer of invading ions. These numerical results clearly show the necessity to incorporate the mechano–electrochemical coupling and large deformation in modeling ion exchange, which is uninvolved in previous theoretical models. Moreover, the proposed theoretical model is directly extendable to three‐dimensional scenarios, such as curved or foldable ultrathin glass panels.