This paper is concerned with the application of a novel engineering method we have recently devised to estimate fatigue lifetime of aluminium welded joints subjected to constant-amplitude uniaxial and multiaxial fatigue loading. The assessment technique employed in the present study is based on the use of the so-called Modified Wöhler Curve Method (MWCM), a conventional critical plane approach, applied in conjunction with the theory of critical distances (TCD). In more detail, the MWCM was initially calibrated by using two standard curves: the first one, stated by Eurocode 9, suitable for assessing ground butt welds subjected to uniaxial loading, whereas the second one, suggested by the International Institute of Welding (IIW), suitable for estimating fatigue strength of aluminium welded details loaded in torsion. Subsequently, a unifying critical distance value to be used to assess aluminium welded joints was calculated by taking full advantage of the master curve supplied by the notch-stress intensity factor (N-SIF) approach and obtained by summarising the uniaxial fatigue strength of cruciform aluminium welded details characterised by different absolute dimensions. Finally, the accuracy and reliability of the devised method was systematically checked by means of several experimental results taken from the literature and generated by testing a variety of welded geometries subjected to uniaxial as well as to multiaxial fatigue loading. Such an extensive validation exercise allowed us to prove that our approach is successful in estimating fatigue damage in aluminium welded details, resulting in predictions mainly falling within the two reference scatter bands adopted to calibrate the method itself. Such a high accuracy level is very promising, especially in light of the fact that our engineering approach can be applied to assess real aluminium welded components by directly post-processing simple linear-elastic finite element (FE) models.