Previous studies have demonstrated the effectiveness of strengthening with pre-stressed carbon-fiber reinforced polymer (CFRP) composites to increase the lifetime of cracked steel members. In some cases, complete crack arrest has been observed. This study aims to present a method that can estimate the minimum required prestressing that would result in a complete crack arrest in steel I-beams. Analytical and numerical models based on linear elastic fracture mechanics (LEFM) were developed and verified using a set of experimental results. Three steel I-beams with different crack lengths were strengthened with pre-stressed CFRP composites and later tested under a high-cycle fatigue loading regime. It was shown that the pre-stressed CFRP composites could result in a crack closure mechanism, in which the crack surfaces remained closed even under large external loads. Furthermore, it was shown that by considering the stiffness of the CFRP in the analytical formulation, the amount of prestressing required to arrest the fatigue crack growth can be reduced.