This work focuses on the microstructural development and the structure-property relations in the Al-Mg AA5xxx Friction Stir Welds (FSWs), highlighting the influence of the basemetal microstructural features (e.g. grains, intermetallic particles, and stored energy), and the role of the basemetal processing route on the weld microstructure. Qualitative and quantitative microstructural investigations using optical, scanning and transmission electron microscopy were performed on 5251-H34 and O, 5754-O (prepared by direct chill and continuous casting), and 5083-H116 FSWs. It was found that the spatial distribution of the constituent intermetallic particles and dispersoids in the basemetal influenced the microstructural features associated with the FSWs (e.g. the formation of the onion rings structure). The starting temper did not influence the spatial hardness distribution nor the microstructure in the TMAZ. However, the microstructural heterogeneities associated with the onion rings structure were observed to create abrupt increases in the hardness, compared to a roughly uniform TMAZ hardness in the welds that did not show clear onion rings structure. In general, grain boundary strengthening is the main strengthening mechanism, although an additional contribution from Orowan strengthening can be found.