Möngke was born on January 11, 1209, as the eldest son of Genghis Khan's teenaged son Tolui and Sorghaghtani Beki. Teb Tengri Khokhcuu, a shaman, claimed to have seen in the stars a great future for the child and bestowed on him the name Möngke, "eternal" in the Mongolian language. His uncle Ögedei Khan's childless queen Angqui raised him at her orda (nomadic palace). Ögedei instructed Persian scholar Idi-dan Muhammed to teach writing to Möngke.
On his way back home after the Mongol conquest of Khwarezmia, Genghis Khan performed a ceremony on his grandsons Möngke and Kublai after their first hunting in 1224 near the Ili River. Möngke was fifteen years old, and with his brother, Kublai, killed a rabbit and an antelope. Their grandfather smeared fat from the killed animals onto their middle fingers following the Mongol tradition.
In 1230, Möngke went to war for the first time, following Ögedei Khan and his father Tolui into battle against the Jin dynasty. Tolui died in 1232, and Ögedei appointed Sorghaghtani head of the Toluid appanage. Following the Mongol custom, Möngke inherited at least one of his father's wives, Oghul-Khoimish of the Oirat clan. Möngke deeply loved her and gave special favor to her elder daughter, Shirin.
Ögedei dispatched him along with his relatives to attack the Kipchaks, Russians, and Bulgars in the west in 1235. When the most formidable Kipchak chief, Bachman, fled to an island in the Volga delta. Möngke crossed the river and captured him. When he ordered Bachman to bend down on his knees, Bachman refused and was executed by Möngke's brother Bujek. Möngke also engaged in hand-to-hand combat during the Mongol invasion of Rus'. While his cousins, Shiban and Büri, went to Crimea, Möngke and Kadan, a son of Ögedei, were ordered to reduce the tribes in the Caucasus. The Mongols captured the Alan capital Maghas and massacred its inhabitants. Many chiefs of the Alans and Circassians surrendered to Möngke. After the invasion of Eastern Europe, Möngke would bring them back to Mongolia. He also participated in the Siege of Kiev (1240). Möngke was apparently taken by the splendour of Kiev and offered the city surrender, but his envoys were killed. After Batu's army joined Möngke's soldiers, they sacked the city. He also fought alongside Batu at the Battle of Mohi. In the summer of 1241, before the premature end of the campaign, Möngke returned home after his uncle Ögedei recalled him in the winter of 1240–41. However, Ögedei died.
In 1246, Temüge, Genghis Khan's sole remaining brother, unsuccessfully tried to seize the throne without confirmation by a kurultai. The new Khagan Güyük entrusted the delicate task of trying the Odchigin ("keeper of the hearth" - a title given to both of Genghiz's younger brothers) to Möngke and Orda Khan, the eldest brother of Batu. Güyük eventually died en route to the west in 1248 and Batu and Möngke emerged as main contenders. .