Temüjin is born on the steppes (grasslands) of what is now Mongolia. He is the son of Yesügei, a member of the royal Borjigin clan of the nomadic Mongol people."

Temüjin grew up with three brothers, Qasar, Hachiun, and Temüge, one sister, Temülen, and two half-brothers, Behter and Belgutei. As common to nomads in Mongolia, Temüjin's early life was difficult.

At age nine, his father arranged a marriage for him and delivered him to the family of his future wife Börte of the tribe Khongirad. Temüjin was to live there serving the head of the household Dai Setsen until the marriageable age of 12. While heading home, his father ran into the neighboring Tatars, who had long been Mongol enemies, and they offered his father food under the guise of hospitality, but which instead poisoned him. Upon learning this, Temüjin returned home to claim his father's position as chief, but the tribe refused him and abandoned the family, leaving it without protection.

For the next several years, the family lived in poverty, surviving mostly on wild fruits, ox carcasses, marmots, and other small game killed by Temüjin and his brothers. During this time, Temüjin's mother, Hoelun, taught him about Mongol politics, including the lack of unity between the different and the need for arranged marriages to solidify temporary alliances, and strong alliances to ultimately ensure the stability of Mongolia. Over time, Temüjin's older half-brother Behter began to exercise power as the eldest male in the family, creating tension in the family that boiled over during one hunting excursion by the men of the family and resulted in the death of Behter at the hands of Temüjin and his brother Qasar.

In a raid around 1177, Temüjin was captured by his father's former allies, the Tayichi'ud, enslaved and, reportedly, humiliated by being shacked in a cangue (a form of portable stocks). With the help of a sympathetic guard, he escaped from the ger (yurt) at night by hiding in a river crevice. The escape earned Temüjin a reputation. Soon, Jelme and Bo'orchu joined forces with him, and they and the guard's son Chilaun eventually became generals of Genghis Khan.

Temüjin began his ascent to power by offering himself as an ally (or, according to other sources, a vassal) to his father's anda (sworn brother or blood brother) Toghrul, who was Khan of the Keraites. This relationship was first reinforced when Börte was kidnapped by Merkits in around 1184. To win her back, Temüjin called on the support of Toghrul, who offered 20,000 of his Keraite warriors and suggested that Temüjin involve his childhood friend Jamukha, who was Khan of his own tribe, the Jadaran.

As Jamukha and Temüjin drifted apart in their friendship, each began consolidating power and they became rivals. Jamukha supported the traditional Mongolian aristocracy, while Temüjin followed a meritocratic method, and attracted a broader range and lower class of followers. Following his earlier defeat of the Merkits, and a proclamation by the shaman Kokochu that the Eternal Blue Sky had set aside the world for Temüjin, Temüjin began rising to power. In 1186, Temüjin was elected khan of the Mongols.

Jamukha assembled 30,000 men and moved in an arc from the north to flank Temüjin's position. The two forces were evenly matched but Temüjin's side suffered slightly worse than Jamukha, and was forced to retreat to a defensible pass called Jerene near the Onon River. Despite Jamukha's victory, his harsh treatment of captives disgusted his allies so much that they defected to Temüjin, bringing with them 10,000 men. With less than 20,000 men at his side, Jamukha was no longer able to challenge Temüjin on the upper Kherlen River, and retreated further east

Temüjin defeats the Naimans, Merkits, and Jamukha; Merkit leaders and Jamukha flee to the Altai Mountains. Temüjin pursues Jamukha and defeats him in several battles. Eventually Jamukha's allies betray him and turn him over to Temüjin, who kills him by breaking his back.

Using his rival Nilga Senggum's temporary refuge in Western Xia as a pretext, Temujin launched a raid against the state in 1205 in the Edsin region. The Mongols plundered border settlements and one local Western Xia noble accepted Mongol supremacy.

In 1209, the Genghis undertook a larger campaign to secure the submission of Western Xia. After defeating a Tangut army led by Kao Liang-Hui outside Wuhai, Genghis captured the city and pushed up along the Yellow River, defeated several cities, and besieged the capital, Yinchuan, which held a well-fortified garrison of 150,000. Emperor Li Anquan, still threatened by the Mongols and receiving no relief from the Jin dynasty, agreed to submit to Mongol rule, and demonstrated his loyalty by giving a daughter, Chaka, in marriage to Genghis and paying a tribute of camels, falcons, and textiles.

The Battle of Yehuling, literally the Battle of Wild Fox Ridge, was a major decisive battle fought between the Mongol Empire and Jurchen-led Jin dynasty during the first stage of the Mongol conquest of the Jin dynasty. The battle was fought between August and October 1211 at Yehuling (lit. "Wild Fox Ridge"), which is located northwest of present-day Wanquan District, Zhangjiakou, Hebei Province. The battle concluded with a total Mongol victory that allowed them to overrun and conquer the northern part of the Jin. It also hastened the weakening and decline of the Jin dynasty.

The Battle of Zhongdu (present-day Beijing) was a battle in 1215 between the Mongols and the Jurchen Jin dynasty, which controlled northern China. The Mongols won and continued their conquest of China. The battle for Beijing was long and tiresome, but the Mongols proved to be more powerful as they finally took the city on 1 June 1215, massacring its inhabitants. This forced Jin Emperor Xuanzong to move his capital south to Kaifeng, and opened the Yellow River valley to further Mongol ravages. Kaifeng also fell to the Mongols after a siege in 1232.

In 1218, after requesting Muhammad II of Khwarazm not to aid Kuchlug, Genghis Khan dispatched general Jebe with two tumens (20,000 soldiers), along with the Uyghur Barchuk (who was Genghis Khan's son-in-law) and possibly also Arslan Khan, ruler of the Karluk city Qayaliq and another son-in-law of Genghis Khan, to deal with the Qara Khitai threat, while sending Subutai with another two tumens on a simultaneous campaign against the Merkits.

The two armies traveled alongside each other through the Altai and Tarbagatai Mountains until arriving at Almaliq. At that point, Subutai turned southwest, destroying the Merkits and protecting Jebe's flank against any sudden attacks from Khwarazm. Jebe relieved Almaliq, then moved south of Lake Balkash into the lands of the Qara Khitai, where he besieged the capital of Balasagun. There, Jebe defeated an army of 30,000 troops and Kuchlug fled to Kashgar. Taking advantage of the unrest fomenting under Kuchlug's rule, Jebe gained support from the Muslim populace by announcing that Kuchlug's policy of religious persecution had ended.

When Jebe's army arrived at Kashgar in 1217, the populace revolted and turned on Kuchlug, forcing him to flee for his life. Jebe pursued Kuchlug across the Pamir Mountains into Badakhshan in modern Afghanistan. According to Ata-Malik Juvayni, a group of hunters caught Kuchlug and handed him over to the Mongols, who promptly beheaded him. The Mongols fully conquered the former territories of the Qara-Khitans in 1220.

With the death of Kuchlug, the Mongol Empire secured control over the Qara Khitai. The Mongols now had a firm outpost in Central Asia directly bordering the Khwarazm Empire. Relations with the Khwarazms would quickly break down, leading to the Mongol invasion of that territory. It was not originally the intention of the Mongol Empire to invade the Khwarezmid Empire, and according to Juvaini, Genghis Khan had originally sent the ruler of the Khwarezmid Empire, Sultan Muhammad Aladdin, a message seeking trade and greeted him as his neighbor.

However, the Governor of Otrar refused to receive the mission and had all 450 of them killed, with permission from the Sultan. Upon hearing of this atrocity months later, Genghis Khan flew into a rage and used the incident as a pretext for invasion. The Mongol invasion of Central Asia however would entail the utter destruction of the Khwarezmid Empire along with the massacre of much of the civilian population of the region.

According to Juvaini, the Mongols ordered only one round of slaughter in Khwarezm and Transoxiana, but systematically exterminated a particularly large portion of the people of the cities of Khorasan. This earned the Mongols a reputation for bloodthirsty ferocity that would mark the remainder of their campaigns.

• Genghis Khan takes Bukhara and places Yelü Ahai in control of Transoxiana
• Genghis Khan takes Samarkand and Muhammad II of Khwarezm flees to Nishapur; Genghis Khan dispatches Jebe and Subutai to destroy the sultan
• Jebe and Subutai take Balkh and capture Muhammad II of Khwarezm's mother Terken Khatun and family in the Zagros Mountains

Following the Mongol invasion of Khwarezm, Jalal ad-Din was forced to flee towards the Hindu Kush, where he began to muster additional troops to face the Mongols. With the arrival of over 30,000 Afghan warriors. His strength reportedly was between 30,000 and 60,000 men. Genghis Khan sent his chief justice Shikhikhutag to hunt down Jalal al-Din, but only gave the rookie general 30,000 troops.

Shikhikhutag was overconfident after the continuous Mongol successes, and he quickly found himself on the back foot against the much more numerous Khwarezmian force. The battle took place in a narrow valley, which was unsuitable for the Mongol cavalry. Jalal al-Din had mounted archers, whom he ordered to dismount and fire on the Mongols. Because of the narrow terrain, the Mongols could not use their normal tactics.

To deceive the Khwarezmians, Shikhikhutag mounted straw warriors on spare remounts, which may have spared him from a killing stroke, but he was still driven off in defeat losing over half his army.

Jalal ad-Din positioned his army of at least thirty thousand men in a defensive stance against the Mongols, placing one flank against the mountains while his other flank was covered by a river bend.The initial Mongol charge that opened the battle was beaten back. Jalal al-Din counterattacked, and nearly breached the center of the Mongol army. Genghis then sent a contingent of 10,000 men around the mountain to flank Jalal ad-Din's army. With his army attacked from two directions and collapsing into chaos, Jalal al-Din fled across the Indus river.

The Mongols made their first appearance in the Georgian possessions when this latter kingdom was still in its zenith, dominating most of the Caucasus. First contact occurred early in the fall of 1220, when approximately 20,000 Mongols led by Subutai and Jebe pursued the ousted Shah Muhammad II of the Khwarazmian dynasty to the Caspian Sea. With the consent of Genghis Khan, the two Mongol generals proceeded west on a reconnaissance mission. They thrust into Armenia, then under Georgian authority, and defeated some 10,000 Georgians and Armenians commanded by King George IV "Lasha" of Georgia and his atabeg (tutor) and amirspasalar (commander-in-chief) Ivane Mkhargrdzeli at the Battle of Khunan on the Kotman River. George was severely wounded in the chest.

Though subjugated under the Mongols, the Tangut Dynasty of Xi Xia refuses to lend military support to the campaign against the Khwarzin Dynasty, instead going into open rebellion. After defeating the Khwarzins, Genghis Khan immediately takes his army back to Xi Xia and begins a string of victories over the Tanguts. After victory, he orders the execution of the Tanguts, thereby putting an end to their dynasty. Genghis ordered his generals to systematically destroy cities and garrisons as they went.

The exact cause of his death remains a mystery, and is variously attributed to being killed in action against the Western Xia, illness, falling from his horse, or wounds sustained in hunting or battle. According to The Secret History of the Mongols, Genghis Khan fell from his horse while hunting and died because of the injury. He was already old and tired from his journeys.

After he died, his body was returned to Mongolia and presumably to his birthplace in Khentii Aimag, where many assume he is buried somewhere close to the Onon River and the Burkhan Khaldun mountain (part of the Kentii mountain range). According to legend, the funeral escort killed anyone and anything across their path to conceal where he was finally buried.