A native of Juvayn in Khorasan, Shams al-Din belonged to the namesake Juvaynis, a Persian family of officials and scholars, that claimed ancestry from al-Fadl ibn al-Rabi' (d. 823/4), who had served in high offices under the Abbasid caliph Harun al-Rashid (r. 786–809).

[1] Shams al-Din was also the younger brother of the historian Ata-Malik Juvayni, who wrote the Tarikh-i Jahangushay ("History of the World Conqueror").

The reason behind his rising influence may have been his friendship with Nasir al-Din al-Tusi, the famed scholar and Hulagu's close advisor, and his marriage to the daughter of the Mongol governor of Khorasan, Arghun Aqa.

Shams al-Din's influence soon increased even further; he received the governorship of Tabriz and played a prominent role in rebuilding Iran, which had suffered greatly from the Mongol conquest.

[1] Shams al-Din was also closely linked with the local vassal states of the Ilkhanids, such as the Kartids of Herat, the Qutlugh-Khanids of Kerman, the Salghurids of Fars, and the Hazaraspids of Luristan.

However, three years later, Majd al-Molk made a more successful attempt; he not only once again accused the brothers of collaborating with the Mamluks, but also stealing hefty amount of riches from the treasury.