Additively manufactured (AM) aluminum components have gained much attention, as the complex thermal history of parts produced by laser-based powder bed fusion of metals (PBF-LB/M) results in unique microstructural features as well as superior mechanical properties. Welding and joining of components produced by additive manufacturing and conventional processes hereby offers new opportunities in structural design. For a reliable use of such hybrid components, the microstructural evolution and mechanical behavior of welded joints under static and cyclic loading have to be investigated. In the present study, AlSi10Mg specimens were fabricated using two different manufacturing processes, i.e., PBF-LB/M and casting, and subsequently welded by means of friction stir welding (FSW). Different thermal cycles during casting, additive manufacturing and welding, in combination with significant plastic deformation in the weld zone, led to significant microstructural differences. Tensile and strain-controlled low-cycle fatigue tests, assisted by digital image correlation, were conducted on similar and dissimilar welded joints in different combinations, namely AM-cast and cast-cast. Material strength of dissimilar welded joints was found to be governed by the cast material region, which is characterized by a particularly coarse microstructure, resulting in inferior hardness and tensile properties. Nevertheless, FSW can be used to create defect-free hybrid joints and large graded parts, as long as the differing local material strengths are taken into account.