On September 8, 1897, at the age of 76, James Longstreet married Helen Dortch at the Governor's mansion in Atlanta, much to his children’s disapproval. He and his 34-year-old bride honeymooned briefly near Atlanta and later took a trip to Mexico. 

Helen would be instrumental in the effort to salvage her husband's reputation. She outlived "Old Pete" by fifty-nine years, spending much of that time defending his reputation. Helen Longstreet was "as combative as Old Pete himself when responding to a slight against his good name," according to the historian Carol Reardon. 

When she visited the Gettysburg battlefield for a fiftieth-anniversary commemoration, she insisted that Pickett's Charge, in fact, "was Longstreet's." Mrs. Longstreet wrote in the New York Times of imagining her husband's feelings of "dumb agony as he looked upon the marching columns and knew that it was their death march." 

During World War II (1939–1945), Helen Longstreet, then in her eighties, worked as a riveter, and in 1950, she ran unsuccessfully for governor of Georgia.

As the years passed, Longstreet became bitter, and his attempts to "set the record straight" made the situation worse. He was naive in many ways. He failed to follow his uncle's advice not to anger people by submitting controversial letters to newspapers. He didn't anticipate extreme, long-lasting Southern hatred toward him, nor that there would be consequences for supporting Grant, becoming a Republican, and accepting political appointments.

In defense of his criticism of Lee's tactical offensive at the Battle of Gettysburg, which Longstreet maintained resulted in the needless death of thousands of Confederate troops during Pickett's Charge, the former general published his memoirs, From Manassas to Appomattox: Memoirs of the Civil War in America, in 1896.