A novel methodology to predict the tensile properties of viscoelastic materials as a synergistic function of temperature and loading/displacement rate is presented in this paper. The model is based on the amount of energy dissipated during tensile loading, requiring less input data compared to existing models. The deformation activation energy was measured and accordingly an energy-based temperature shift factor was suggested. Additionally, to describe the material's rate dependency, two new types of energy-based loading/displacement rate shift factors based on the Eyring and Cooperative models were derived. The shift factors were implemented into the theory of viscoelasticity to predict the material tensile behavior at different temperatures and loading/displacement rates. In addition, a new failure criterion was suggested to predict the mechanical strength of the tested materials through the definition of a critical energy level, which determined the maximum energy that a material would be able to dissipate before failure. The model was applied to two different viscoelastic materials; an epoxy adhesive, and an angle-ply glass/epoxy fiber-reinforced polymer (GFRP) composite, two common viscoelastic materials used in the structural and construction industries. To determine the accuracy of the model, the results obtained from the model were compared to the results obtained from experiments. It was observed that the suggested methodology well predicted the stress-strain curves of the tested materials in the linear region and yield stress up to the post-yield region and failure point. In addition, the strength surface of the studied materials, which showed the synchronous effect of temperature and loading/displacement rate on the ultimate tensile strength in a 3-dimensional representation, was introduced.