The deception was discovered during restoration work on the painting when the curator of the Abraham Lincoln Library and Museum had it removed from the governor's mansion during rehabilitation work there.
"It was supposedly a gift Mary Lincoln planned to give to her husband, but then he was assassinated and she became a widow before she could present it to him," curator James Cornelius told the Chicago Tribune Saturday.
Restorers noticed the signature on the painting seemed to have been added later and investigation revealed the original subject appeared not to have been Mary Lincoln.
"It's some anonymous woman," Cornelius said.
The painting was sold to the Lincoln family in the late 1920s for between $2,000 and $3,000, Cornelius said.
The museum said it plans to display the painting for a while, he said.
"It doesn't have the backstory we thought it had, but in a way it has a new backstory that's quite intriguing."