This paper summarizes an attempt to devise an engineering method suitable for predicting fatigue lifetime of metallic materials subjected to both proportional and nonproportional multiaxial cyclic loadings. The proposed approach takes as a starting point the assumption that the plane experiencing the maximum shear strain amplitude (the so-called “critical plane”) is coincident with the micro-/mesocrack initiation plane. In order to correctly account for the presence of both nonzero mean stresses and nonzero out-of-phase angles, the degree of multiaxiality/nonproportionality of the stress state damaging crack initiation sites is suggested here to be evaluated in terms of the ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio is used then to define nonconventional Manson–Coffin curves, whose calibration is done through two strain-life curves generated under fully reversed uniaxial and fully reversed torsional fatigue loadings, respectively. The accuracy and reliability of our approach were systematically checked by using approximately 350 experimental data taken from the technical literature and generated by testing 13 different materials under both in-phase and out-of-phase loadings. Moreover, the accuracy of our criterion in estimating lifetime in the presence of nonzero mean stresses was also investigated. Such an extensive validation exercise allowed us to prove that the fatigue life estimation technique formalized in the present paper is a reliable tool capable of correctly evaluating fatigue damage in engineering materials subjected to multiaxial cyclic loading paths.