Family members said she died Friday at a nursing home in La Porte City, Iowa.
Mullen wrote "Unfriendly Fire: A Mother's Memoir" after her son Michael died at age 25 when a U.S. artillery shell fell short and killed him on Feb. 18, 1970, near the South Vietnamese village of Tu Chanh.
"This is the first book you've got from the family side of a Vietnam story," Mullen told the Associated Press in a 1995 interview before the book was released.
"All you've read everywhere is the blood and the guts," she said. "But you haven't had anything coming out of what went on as far as the family, as far as brothers and sisters and mothers and dads."
Almost from the day Mullen and her husband, Gene, who died in 1986, learned that Michael had been killed, she tried to get more information about their son's death from the U.S. military. Her full-page ad in the Des Moines Register protesting the war and marches in antiwar demonstrations put her on par with more notable protesters of the day.
Her other son, John Mullen, said Sunday that he doesn't know if his mother was ever satisfied with the information she tracked down, "but she came to terms with it."
"She brought to the forefront the idea of friendly fire. It was a term that never got much play until that time," he said. "I think she'll be remembered as somebody who asked a lot of questions, somebody who wouldn't take a pat answer, somebody who would stand up for something she believed in. You need those types of people."
Peg Mullen received many letters, phone calls and notes from other parents who had lost sons and from combat veterans who told her they knew and had served with Michael.
Her book includes 40 letters from Michael, along with an account of her conversation one night in 1989 with the man who told her he had fired the fatal shell. It also lambasts Norman Schwarzkopf, the Gulf War general who was Michael's battalion commander in Vietnam.
In addition to her son John, Mullen is survived by daughters Patricia Hulting of Des Moines and Mary DeJana of Kalispell, Mont.