Güyük received military training and served as an officer under his grandfather Genghis Khan and later his father Ögedei Khan (after the death of Genghis in 1227). He married Oghul Qaimish of the Merkit clan. 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, in a few months. After the death of Güyük‘s uncle Tolui, Ögedei proposed that Sorghaghtani, the widow of Tolui, marry his son Güyük. Sorghaghtani declined, saying that her prime responsibility was to her own sons.
The Mongols outside Vladimir
presumably demanding submission before its sack.
Güyük participated in the invasion of Russia and Central Europe in 1236–1241 with other Mongol princes, including his cousin Batu and half-brother Kadan. He led his corps in the Siege of Ryazan and the lengthy siege of the Ossetian capital Maghas. During the course of the conquest, Güyük quarreled violently with Batu at the victory banquet and screamed at him, "Batu is just an old woman with a quiver". Güyük and Büri, a grandson of Chagatai, stormed out of the banquet and rode away swearing and cursing. When word reached the Great Khan, they were recalled for a time to Mongolia. Ögedei refused to see them and threatened to have his son Güyük executed. Ögedei calmed down and finally admitted Güyük into his ger. Ögedei criticized Güyük, "Do you think that the Russians surrendered because how mean you were to your own men. ...Because you captured one or two warriors, you think that you won the war. But you didn't capture even a single kid goat." Ögedei reprimanded his son harshly for fighting within the family and for mistreating his soldiers. Güyük was dispatched again to Europe.
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.