The 19-year-old male was killed by Maasai morans, or warriors, after he wandered into a livestock pen on the outskirts of Amboseli National Park.
The park is renowned for its close-up experiences with free-ranging elephants and other wildlife.
“It was an old lion that had issues … getting prey on its own and livestock is easy prey,” said Paul Jinaro, a spokesman for Kenya Wildlife Service.
“A normal lion would go for wildlife inside the park.”
African lions generally live up to 18 years in the wild, according to conservation group Cats for Africa.
The Kenya Wildlife Service had previously called Loonkiito a “legendary big cat warrior” who had staunchly defended his territory for more than a decade.
“It is with heavy hearts that we share the news of the passing of Loonkiito (2004-2023), the oldest male lion in our ecosystem and possibly in Africa,” the non-profit Lion Guardians said on Facebook.
The lion's death shines a light on a growing issue in Kenya: encounters between wildlife and humans have been on the rise in recent years as urban development encroaches on animal habitats.
“People need to be sensitised on looking for a way to alert us and then we can take animals back to the parks,” Mr Jinaro said.
In 2021, a lion strayed from Nairobi National Park into a densely populated neighbourhood, causing widespread panic.
The park is 7 kilometres away from the Kenyan capital and incidents of wildlife venturing into the bustling city of more than four million people are not uncommon.
In December 2019, a lion killed a man right outside the park, and in March 2016, a lion was shot after injuring a local resident.
Kenya's wildlife population, which includes an estimated 2,500 lions, is under increasing pressure, according to the nation's first ever national wildlife census conducted in 2021.