During his almost two year reign, he reversed some of his mother's unpopular edicts and ordered an empire-wide census; he also held some authority in Eastern Europe, appointing Andrey II as the grand prince of Vladimir and giving the princely title of Kiev to Alexander Nevsky.

According to Giovanni da Pian del Carpine, Güyük was of "medium stature, very prudent and extremely shrewd, and serious and sedate in his manners.

In 1233, Güyük, along with his maternal cousin Alchidai and the Mongol general Tangghud, conquered the short-lived Dongxia Kingdom of Puxian Wannu, who was a rebellious Jin official,[3] in a few months.

Güyük participated in the invasion of Eastern and Central Europe in 1236–1241 with other Mongol princes, including his cousin Batu and half-brother Kadan.

In the meantime, Ögedei had died (in 1241), and his widow Töregene had taken over as regent, a position of great influence and authority that she used to advocate for her son Güyük.

Batu withdrew from Europe so that he might have some influence over the succession, but despite his delaying tactics, Töregene succeeded in getting Güyük elected Khan in 1246.

When Genghis Khan's youngest brother, Temüge, threatened the Great Khatun Toregene in an attempt to seize the throne, Güyük came to Mongolia from Emil to secure his position immediately.

Güyük's enthronement on 24 August 1246, near the Mongol capital at Karakorum, was attended by a large number of foreign ambassadors: the Franciscan friar and envoy of Pope Innocent IV, John of Plano Carpini and Benedict of Poland; Grand Duke Yaroslav II of Vladimir; the incumbents for the throne of Georgia, David VI and David VII; the brother of the king of Armenia and historian, Sempad the Constable; the future Seljuk Sultan of Rum, Kilij Arslan IV; and ambassadors of the Abbasid Caliph Al-Musta'sim and Ala ud din Masud of the Delhi Sultanate.

[7] According to John of Plano Carpini, Güyük's formal election in a great kurultai, or diet of the tribes, took place while his company was at a camp called Sira Orda, or "Yellow Pavilion," along with 3,000 to 4,000 visitors from all parts of Asia and eastern Europe, bearing homage, tribute, and presents.

And all this great assembly came with such baggage as befitted a court; and there came also from other directions so many envoys and messengers that two thousand felt tents had been made ready for them: there came also merchants with the rare and precious things that are produced in the East and the West.When the papal envoy John of Plano Carpini protested Mongol attacks on the Catholic kingdoms of Europe, Güyük stated that these people had slain Mongol envoys in the time of Genghis Khan and Ogedei Khan.

He also claimed that "from the rising of the sun to its setting, all the lands have been made subject to the Great Khan", proclaiming an explicit ideology of world conquest.

[11] Güyük replaced the child khan Qara Hülëgü of the Chagatai Khanate with his favorite cousin Yesü Möngke to secure his position.

Güyük reversed several unpopular edicts of his mother the regent and made a surprisingly capable khan, appointing Eljigidei in Persia in preparation for an attack on Baghdad and the Ismailis and pursuing the war against the Song Dynasty.

Güyük was a strict and intelligent person, though rather morose and sickly, and his bad drinking habit worsened his health.

After the king Gojong of Goryeo refused to move his capital from Ganghwa Island to Songdo, Amuqan's force pillaged the Korean Peninsula until 1250.

Although Batu did not support Güyük's election, he respected the Great Khan as a traditionalist and sent Andrey and Alexander Nevsky to Karakorum in Mongolia in 1247 after their father's death.