The eight-second snippet of footage shows Roosevelt, with braces on his legs apparent beneath his pants, and holding an assistant's arm and the handrail, climbing awkwardly up a ramp to his seat.
It is one of only two known extended film clips showing Roosevelt walking, state officials said.
Susan Corbett, the governor's wife, and members of DeShong's family unveiled the film at the Pennsylvania State Museum.
The discovery came just in time for filmmaker Ken Burns, who is completing a documentary on the Roosevelt family to air on PBS in September.
Burns said he was thrilled to learn of the discovery because it was known that the Secret Service would confiscate cameras whenever someone took images of Roosevelt struggling to move on his own.
"The president wanted to minimize the public's knowledge of the devastating effects polio had had on him," Burns said in a statement. Burns said he would swap out a still photo and use the new footage in his film.
DeShong's daughter, Judith Savastio, donated the film to the archives several months ago. It will be preserved and interpreted and made accessible to the public, state officials said.
Also in the nearly six minutes of footage, taken from the field, are New York Yankees great Lou Gehrig, Athletics and Boston Red Sox star Jimmie Foxx, Detroit Tigers legend Hank Greenberg, and others.
To see the video, go to www.inquirer.com/fdrvideo.