The broadcast by state media Monday from the North Korean capital city of Pyongyang said Kim died of a heart ailment on a train due to a "great mental and physical strain" on Dec. 17 during a "high intensity field inspection."
The announcement said the diagnosis was "fully confirmed" by an autopsy conducted on Dec. 18.
Kim is believed to have suffered a stroke in 2008 and was believed to have had diabetes and heart disease.
The communist country's "Dear Leader" - reputed to have had a taste for women, wine and American movies - appeared relatively energetic in photos and video from recent trips around the country and to China and Russia.
The news came as North Korea prepared for a hereditary succession. Kim Jong Il inherited power after his father, revered North Korean founder Kim Il Sung, died in 1994. In September 2010, Kim Jong Il unveiled his third son, the twenty-something Kim Jong Un, as his successor, putting him in high-ranking posts.
Kim Jong Il had been groomed for 20 years to lead the communist nation founded by his guerrilla fighter-turned-politician father and built according to the principle of "juche," or self-reliance.
The White House press secretary issued a statement Sunday, saying it was closely monitoring the reports of Kim's death.
"The President has been notified, and we are in close touch with our allies in South Korea and Japan," the statement said. "We remain committed to stability on the Korean peninsula, and to the freedom and security of our allies."
The Associated Press contributed to this report.