Moses Lekalau, 35, was walking home in the Maralal safari area north-east of Nairobi when the lion leapt out at him.
Exhausted by his struggle, he lacked the strength to fight off a subsequent attack by hyenas.
Mr Lekalau was eventually rescued by a passing motorist who drove him to hospital. He was airlifted to Nairobi for surgery but died of blood loss.
Mr Lekalau told doctors that it took him half an hour to spear and bludgeon the lion to death.
As a Samburu, he came from a community where it is traditional for boys to kill a lion as a rite of passage on entering manhood.