Gilbert was born in Los Angeles, California, on May 8, 1964, to a newly engaged couple, Kathy Wood and David Darlington, and given up for adoption immediately after birth. She was adopted one day later by actor and comedian Paul Gilbert (born Ed MacMahon, he changed his name to Paul Gilbert to get a Screen Actors Guild card) and his wife, dancer and actress Barbara Crane, the daughter of The Honeymooners creator Harry Crane. The couple later adopted a son, Jonathan, who co-starred on Little House on the Prairie. Gilbert's adoptive parents divorced when she was 8 years old. Barbara Crane Gilbert then married Harold Abeles, and together they had biological daughter Sara Rebecca Abeles (the actress known professionally as Sara Gilbert) on January 29, 1975. On February 13, 1976, Paul Gilbert died. Although 11-year-old Melissa was told that he had suffered a sudden stroke, she found out years later that the 57-year-old had been a VA patient who dealt with constant pain and that he had taken his own life. The marriage of Barbara and Harold Abeles later ended in divorce. According to her biography, Gilbert was "lightly raised" in her adoptive mother's Jewish religion (her adoptive father was a Gentile), but did not have any formal religious education or conversion ceremony.