Controlling the size and morphology of primary and eutectic Mg2Si phase via modification and refinement (i.e., Ca and P addition) in Al-Mg2Si-based alloys containing higher percentages of Si and Mg is of great necessity to ensure their high performance. The present investigation is mainly aimed to elucidate the effect of Ca and P on the nucleation and growth pattern of eutectic Mg2Si in high-purity Al-13.1 wt.% Mg2Si near-eutectic alloys with and without the additions of P and Ca. It was found that purity levels of raw starting materials exert a significant influence on the size and morphology of the near-binary eutectic Al-Mg2Si. The presence of P suppresses the precipitation of ternary eutectic Al-Mg2Si-Si. The presence of low levels of Ca promotes the faceted growth of Mg2Si, while at sufficiently higher levels of Ca, Al2Si2Ca intermetallic is formed and the morphology of the Mg2Si phase is changed from fibers/or globules to blocks or plates. The current investigation is of great importance for the further development of high-performance Al-Mg2Si-based cast alloys.