Social activist Margaret (Peg) Mullen was born in 1917, in Pocahontas, Iowa, the daughter of Clair and Josephine Wolfe Goodyear. She graduated from Sacred Heart High School and was employed as a secretary in the US Department of Labor from 1937 to 1944.

In November 1941, she married Oscar Eugene (Gene) Mullen of Waterloo. During World War II, he served in the Army in Des Moines, Iowa; Springfield, Missouri; and Fort Logan, Colorado. After his discharge, the Mullens moved to the 120-acre family farm near La Porte City, Iowa. From 1955 to 1961 Peg Mullen commuted to Waterloo to work in an advertising agency. In addition to working the farm, Gene Mullen was employed as a quality control supervisor at the John Deere Tractor Works in Waterloo.

Peg Mullen participated in community affairs, including the church youth group and 4-H, and in politics as a Democratic delegate to county and state conventions. The Mullens had four children: Michael, Patricia, Mary Margaret, and John Kevin.

Michael Mullen graduated from Don Bosco High School in Gilbertville, Iowa, in 1963 and earned a BA in chemistry from Rockhurst College, a Jesuit school in Kansas City, Missouri, in 1967. He was awarded a fellowship for graduate study at the University of Missouri and studied there for one year. In September 1968, he was drafted into the Army. He completed basic training at Camp Polk, Louisiana, and attended accelerated Non-Commissioned Officers (NCO) training at Fort Benning, Georgia. In September 1969, he was sent to Vietnam and served with the 198th Light Infantry Americal division near Chu Lai. Michael Mullen was killed on February 18, 1970, by shrapnel discharged by United States artillery, also known as "friendly fire."

Peg Mullen had opposed the Vietnam War before her son was drafted, but after his death she became a public critic of the war. She became widely known for urging the Pentagon to release documents that pertained to the circumstances surrounding her son's death and her involvement in the anti-war protest movement. With their son's military gratuity, the Mullens placed two ads in "The Des Moines Register". The first ran on April 12, 1970. It consisted of 714 crosses representing those Iowans reported killed in Vietnam. Another ran on April 26, with 719 crosses. The response to the ads was dramatic and Peg Mullen soon became a national figure. She received thousands of letters of support from throughout the country; gave interviews for radio, television, and newspapers; counseled families; traveled to Washington to protest the war and spoke frequently at anti-war rallies. The Mullens were one of the families featured in a film, "And Another Family for Peace".

In 1972, Mullen served as a McGovern delegate to the Democratic National Convention. In that same year she was nominated Iowa Mother of the Year. The story of the Mullens' attempts to obtain the facts from the Pentagon about Michael's death attracted the interest of journalist C.D.B. Bryan, who first visited the Mullens in La Porte City in 1971. Five years later, Bryan's three-part series appeared in The New Yorker (March 1, 8, and 15, 1976). The New Yorker series was later expanded and published as Friendly Fire (Putnam, 1976). In 1979, the book was adapted for a television movie starring Carol Burnett and Ned Beatty. The show received six Emmy awards. After the publication of the book and the television production, Mullen received waves of mail, telephone calls, and requests for interviews.

In 1976, Gene Mullen suffered a massive heart attack and received a medical retirement from John Deere. Later that year, the Mullens moved to Brownsville, Texas. Gene Mullen died in July 1986. Peg Mullen later moved back to Iowa, where she continued her anti-war activism by speaking out against US wars in the Persian Gulf and Iraq. She published Unfriendly Fire: A Mother's Memoir (1995), in which she recalls her activities in the years following her son's death in Vietnam. In 1997, Mullen was inducted into the Iowa Women's Hall of Fame. Peg Mullen passed away in La Porte City, Iowa, on October 2, 2009 at the age of 92.