Veterinarians suspect the young orangutan who was rejected by his own mother died of pneumonia, but a necropsy is planned to confirm the cause of death.
Muhal had been at the Milwaukee County Zoo since 2008, where he lived with a surrogate grandmother who raised him until she died.
"He was doing very well after a really rough start to life," Zoo Director Chuck Wikenhauser told the Milwaukee Journal Sentinel. "He was a showoff. He knew he was popular."
Muhal was seen as an ambassador for his endangered species.
Zookeepers noticed Muhal was lethargic Thursday and he was found dead on Saturday.
Orangutans can live well into their 30s.