The aim of the present paper is to systematically investigate the accuracy of the so-called Modified Wöhler Curve Method (MWCM) in estimating fatigue lifetime of engineering components damaged by in-service variable amplitude multiaxial load histories. In more detail, the MWCM, which is a bi-parametrical critical plane approach, postulates that initiation and Stage I propagation of fatigue cracks occur on those material planes experiencing the maximum shear stress amplitude (this being assumed to be always true independently from the degree of multiaxiality of the applied loading path). Further, the fatigue damage extent is hypothesised to depend also on the maximum stress perpendicular to the critical plane, the mean normal stress being corrected through the so-called mean stress sensitivity index (i.e., a material constant capable of quantifying the sensitivity of the assessed material to the presence of superimposed static stresses). To extend the use of the above criterion to those situations involving variable amplitude loadings, the MWCM is suggested here as being applied by defining the critical plane through that direction experiencing the maximum variance of the resolved shear stress: since the resolved shear stress is a monodimensional quantity, stress cycles are directly counted by the classical Rain-Flow method. In the present investigation, the overall accuracy of the MWCM in estimating high-cycle fatigue strength was checked through several experimental results taken from the literature and generated, under variable amplitude multiaxial fatigue loading, by testing plain samples of commercial steels. Such systematic validation exercise allowed us to prove that the MWCM is highly accurate, resulting in estimates falling within the error scatter bands associated with the experimental data used to calibrate the method itself.