Arthur H. Bremer, 57, left the prison before sunrise, said spokesman Mark A. Vernarelli.
Wallace, a fiery segregationist during the 1960s, was wounded on May 15, 1972, during a campaign stop in Laurel, Md. He abandoned his bid for the Democratic nomination and spent the rest of his life in a wheelchair.
Bremer, a former Milwaukee busboy and janitor, was convicted of attempted murder and sentenced to 53 years. He had been held at the medium-security Maryland Correctional Institution near Hagerstown, about 70 miles from Baltimore, since 1979.
In journal entries that came out at his trial, Bremer wrote that he had stalked President Richard M. Nixon and wanted to make a "statement of my manhood for the world to see." He later switched to Wallace, a less well-guarded target than a sitting president, reports the Baltimore Sun.
Wallace was best known for standing defiantly at the all-white University of Alabama in a symbolic face-off with the Justice Department as the National Guard stood by and two black students enrolled in 1963.
By 1972, he had tempered his racist rhetoric and adopted a more subtle approach, denouncing federal courts over the "involuntary busing" of schoolchildren to meet desegregation orders and pledging a return to a "law and order" society.