For 32 years, a portrait of a serene Mary Todd Lincoln hung in the governor’s mansion in Springfield, Ill., signed by Francis Bicknell Carpenter, a celebrated painter who lived at the White House for six months in 1864.

The story behind the picture was compelling: Mrs. Lincoln had Mr. Carpenter secretly paint her portrait as a surprise for the president, but he was assassinated before she had a chance to present it to him.

Now it turns out that both the portrait and the touching tale accompanying it are false.

The canvas, which was purchased by Abraham Lincoln’s descendants before being donated to the state’s historical library in the 1970s, was discovered to be a hoax when it was sent to a conservator for cleaning, said James M. Cornelius, the curator of the Lincoln library and museum in Springfield. The museum is planning to present its findings at a lecture on April 26.

“It was a scam to defraud the Lincoln family,” Mr. Cornelius said.

The Lincolns were not the only ones fooled. Ever since The New York Times announced the portrait’s discovery in 1929, on Feb. 12, Lincoln’s birthday, historians and the public have assumed it depicted Mary Todd Lincoln. It was reproduced in The Chicago Tribune and National Geographic, and versions of it still illustrate at least two biographies, including the latest paperback edition of Carl Sandburg’s 1932 “Mary Lincoln: Wife and Widow.”