Jamukha was born in the Jadaran, a sub-tribe of the Khamag Mongol confederation, and was an anda (i.e. blood brother) to Temüjin.
In 1201, the leaders of the thirteen remaining tribes hostile to Temüjin (among them the Merkits, Tatar, and Naimans) and the Mongol tribes not allied with him (Jadaran, Taichuud, and others) assembled a kurultai and elected Jamukha as Gur-khan, universal ruler, a title used by the rulers of the Kara-Khitan Khanate.
For example, he did not recruit shepherds who lacked tribal status in the Mongol tradition, which allowed Temüjin to recover from a series of military defeats inflicted by Jamukha and to emerge victorious.
Jamukha was given a choice to live and join Temüjin, but he instead requested to die in his blood-brother's hands, and thus was soon after executed "without spilling any blood".
In The Conqueror, the screenplay depicts him as unfailingly loyal and subordinate to Temujin, but ends with him insisting on the bloodless execution (when Temujin swears to grant him any favor he requests), as opposed to the 1965 film, which depicts them as lifelong rivals and enemies who both perish in a climactic duel.