Cobell died Sunday at a hospital in Great Falls, Mont., of complications from cancer, her spokesman Bill McAllister announced.
When she took over as treasurer of the tribe in 1976 she found herself in charge of an accounting system "in total chaos," she told The Times in 2002.
As Cobell attempted to unravel the books, she could make neither "hide nor hair of the trust accounts," she later said, referring to trusts that had been set up as part of the 1887 Dawes Act.
The act tried to erode the tribal system by granting parcels of land to individual Native Americans, but not allowing them to control their new property. Instead, the land was placed in trust with the promise that owners would be paid royalties for oil and gas, grazing or recreational leases.
Yet the Indians received little or no payment, The Times reported in 2009.
Cobell approached the Boulder, Colo.-based Native American Rights Fund about filing a class-action lawsuit against the Interior and Treasury departments, and she was named as lead plaintiff when the suit was filed in 1996. The suit contended that the Dawes Act arrangement allowed U.S. officials to systematically steal and squander royalties intended for Native Americans.
"It's just such a wrong that if I didn't do something about it I'm as criminal as the government," Cobell told the Associated Press in 1999.
Just this June, a federal judge approved the $3.4-billion settlement, the largest payment Native Americans have ever received from the U.S. government.
It provides a $1,000 cash payment to every individual who has a trust account and $2 billion for the federal government to buy back the land parcels, The Times reported when the settlement was reached in 2009. Cobell was to receive $2 million, according to the AP.
In deciding whether to accept the settlement, Cobell said she had to weigh the possibility of winning a greater sum against a harsh reality. The plaintiffs had estimated they were owed as much as $47 billion.
"Time takes a toll, especially on elders living in abject poverty," Cobell said in a 2009 Times interview. "Many of them died as we continued to struggle to settle this suit. Many more would not survive long to see a financial gain, if we had not settled now."
One of eight children, she was born Elouise Pepion on Nov. 5, 1945, on the Blackfeet reservation in Browning, Mont. Her parents owned a 200-acre ranch.
After high school, she attended Great Falls Commercial College and Montana State University in Bozeman but had to leave school after two years to care for her dying mother.
In 1968, Cobell moved to Seattle and worked in the accounting department of a television station. She also met her future husband, Alvin Cobell, a fisherman and fellow member of the Blackfeet tribe.
When her father asked her to come home to help run the struggling family ranch, she returned to the reservation. She had missed the community and the land, Cobell later said.
"Once we got on that ranch, there was no going back," Cobell told the AP. "We just wanted to make sure we held on to our land."
In 1987 Cobell helped found Blackfeet National Bank, the first bank established by a Native American tribe on a reservation.
A decade later she received a $300,000 "genius grant" from the MacArthur Foundation. Surprised by the windfall, she donated most of the money to the class-action suit's legal defense fund.
The cause also received a $4-million assist from businessman J. Patrick Lannan Jr. and his New Mexico-based Lannan Foundation.
"There was something about her that really impressed us," Lannan told The Times in 2002. "I guess it was her ability to describe what it's been like to be an Indian in this sort of thing."
In a 2000 tribal ritual, Cobell was declared a warrior of the Blackfeet Nation and presented with an eagle feather, an honor reserved in modern times almost exclusively for U.S. military veterans.
Cobell is survived by her husband, Alvin; son, Turk; brother Dale Pepion; sisters Julene Kennerly, Joy Ketah and Karen Powell; and two grandchildren.