Peter was born in New York City. His father, Alwin Michaelsen, is a financial consultant and his mother, Gail, was once Alwin's secretary. Gail is the niece of Stork Club owner Sherman Billingsley, and cousin of Glenn Billingsley who was married to Actress Barbara Billingsley (1915–2010). Gail was the one who initially took the children around to auditions. She once took the children to Browns Mills, New Jersey for a commercial commemorating US troops. All five of the children in the family had acting careers when they were young. The oldest of Billingsley's siblings, Dina and Win, had the briefest acting careers working mostly in commercials with brief guest spots on television shows. Peter's elder sister, Melissa, may be best known for her role as Maxx Davis on Me and Maxx. Peter's older brother, Neil, began playing Danny Walton on the daytime soap opera Search for Tomorrow in 1975 and has had numerous roles in commercials and guest shots on TV series. Following his acting career, Neil works in Finance in New York City.

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