Kennedy had been skiing in clear, mild weather about 4:15 p.m. MST with several members of his family when he smashed into a tree, Pitkin County sheriff's authorities said.
Within four minutes, a ski patrol arrived at the scene and provided extensive cardiac treatment, spinal immobilization and respiratory support before transporting Kennedy down the 11,212-foot mountain, authorities said. The intensive care failed to revive him.
Kennedy was transported to Aspen Valley Hospital, located about a mile away, where he was pronounced dead at 5:50 p.m.
Authorities said the incident about 160 miles southwest of Denver "appeared to be accidental."
There were unconfirmed reports that Kennedy and his companions were tossing a football back and forth as they descended the slope.
The holiday fatality was another blow for members of America's most famous and ill-starred political family, who have withstood crises ranging from assassinations to drugs and drinking, from Chappaquiddick to rape charges.
Kennedy, who earlier this year split up with his wife after his alleged affair with a baby sitter came to light, was the second of 11 children born to the former senator and his wife, Ethel, to die under tragic circumstances.
His brother David died in Florida of a drug overdose in 1984, reportedly still depressed over the assassination of his father 16 years earlier.
"Ethel Kennedy and her family are mourning the loss of their beloved Michael, who was fatally injured while skiing with his family in Aspen," said a statement issued by the Aspen Ski Co. "He was a special and wonderful father, son, brother, cousin and friend, and his family would appreciate your prayers during this tragic time."
In a separate statement, his older brother, Rep. Joseph P. Kennedy II (D-Mass.) said, "Michael's death is a terrible tragedy for his children, his wife, Vicki, and his entire family. We will miss him dearly."
A brother plagued by scandal, Michael Kennedy reportedly had conducted a five-year affair with the young daughter of millionaire entrepreneur Paul Verrochi, while she worked as a baby sitter for his three children.
The Boston Globe reported last April that Kennedy's estranged wife, Victoria Gifford Kennedy, daughter of sportscaster Frank Gifford, caught the two in bed two years ago. That report came shortly after Kennedy announced that he was separating from his wife of 16 years.
Recent reports suggested that the couple may have reconciled.
The young woman chose not to file a complaint against Kennedy. But just hours after Norfolk County Dist. Atty. Jeffrey Locke decided to drop an investigation into statutory rape allegations, Kennedy issued a public statement apologizing to the woman's family.
"I intend to do all I can to make up for the serious mistakes I have made," Kennedy said last July, "and to continue the help I need."
Kennedy, a look-alike for his late father, had reportedly sought help for alcohol dependency and in 1996 entered an Arizona facility that treats sex addiction.
Together with his older brother, he started the Citizens Energy Corp., a nonprofit organization bringing heating oil to low-income residents. He also served as campaign manager for his uncle Edward M. Kennedy's intense U.S. Senate reelection campaign in 1994.
As recently as a year and a half ago, Michael Kennedy's name was frequently mentioned as a possible congressional candidate.
The media frenzy sparked by his alleged affair with the family baby sitter was a factor in the withdrawal of Joseph Kennedy--the Democrat who inherited the congressional seat of former House Speaker Thomas P. "Tip" O'Neill--from the 1998 gubernatorial race in Massachusetts.
Kennedy's death on Aspen Mountain's Copper ski run was only the latest incident on slopes that are rated as intermediate, and regarded by locals as potentially dangerous for all but experienced skiers. Weather conditions on Wednesday were clear with mild winds and a 24-inch base of packed powder.
"It's a very difficult mountain--one of the steepest in the valley and definitely not for beginners," said a manager at Aspen's Planet Hollywood nightclub who asked that her name not be used. "Trees are a problem in the glades because they're everywhere."