She married Oscar Eugene "Gene" Mullen (June 27, 1916 - July 1986) in 1941 and lived on a family farm near La Porte City, Iowa, supplementing her income with jobs at J. C. Penney and Santa Claus Industries. The couple had five children: Michael, Daniel (who died two days after being born prematurely), Patricia, Mary Margaret, and John Kevin. Peg Mullen was an active Democrat who was a delegate to the Democratic National Conventions held in 1964, 1968 and 1972. In 1972 she was a McGovern delegate. That same year she was nominated for Iowa Mother of the Year.