In the Mongol system of sacral, divinized kingship, the ritual veneration of Chinggis Khan played a central role as it allowed the Chinggisid successors to maintain the flow of Eternal Heaven’s (Tenggeri) blessing and protection, to the Chinggisid family and thus guaranteed their dynastic stability and well-being. Yet, what happens to this principle of Chinggisid ancestor worship when the Mongols embraced world religions such as Buddhism and Islam? This paper examines the changes to Chinggisid ancestor worship in Mongol-ruled, Ilkhanid Iran (1260-1335), during the two decades following the Mongol court’s official conversion to Islam (1295). Specifically, I explore the relationship between Muslim court debates about shrine veneration alongside the adoption of Muslim, Shiʿi decorative and architectural motifs in the monumental mausoleum of the Muslim Mongol ruler Öljeitü (r. 1304-1316) in the city of Sultaniyya (Iran), which were employed to experiment with a new Islamic royal cult for the Chinggisid Ilkhans. Through these interrelated examples, I demonstrate, first, how Islamic discourses and material representations of shrine veneration were employed to assimilate, convert, and ethicize the Mongols’ immanentist sacral kingship, and second, how these Ilkhanid experimentations relate to the evolution of a new shrine-centered kingship in dialogue with the cult of saints in the post-Mongol Muslim world.