This paper is concerned with a novel elasto-plastic reformulation of the Theory of Critical Distances (TCD) specifically devised to estimate lifetime of notched metallic materials (ferrous and nonferrous) failing in the low/medium-cycle fatigue regime. We used the classic Manson–Coffin and Smith–Topper–Watson approaches, but applied in conjunction with the TCD. We assumed that the material’s critical distance is a constant whose value does not depend on either the sharpness of the notch or on the number of cycles to failure. The accuracy and reliability of the proposed approach was checked by using a number of experimental results generated by testing cylindrical specimens made of En3B, which is a commercial low-carbon steel, and Al6082, which is a conventional aluminum alloy, containing different geometrical features and tested at applied load ratios of and . The resulting predictions of fatigue life were highly accurate, giving estimates falling within an error factor (in lifetime) of about 2. This result is undoubtedly encouraging, especially in light of the fact that the pieces of experimental information needed to calibrate our method can easily be generated by using standard testing equipment, and the necessary stress/strain fields acting on the fatigue process zone can be determined by directly postprocessing elasto-plastic finite element results.