On June 30, 1960, Peter married his 24-year-old secretary Beverly Mater (b. 1935). They had one daughter together, Elizabeth, in January 1961. Peter and Beverly lived with their daughter at the Folger mansion located in Woodside, California until 1974 when they moved to a newly built home on Roberta Drive. The murder of his eldest daughter, Abigail Anne, in Los Angeles, California, on August 9, 1969, was said to have lessened Peter's desire to continue living at the Woodside estate, where she grew up.