Born Merrill Beth Nisker in 1966 in Toronto, Ontario, she attended a private Jewish school. Nisker was a class clown, who "wasn't particularly smart. I was interested in creative performance stuff, but that turned out to be difficult for me because it was all so structured," she told Kitty Empire of The Observer.
Her family was culturally Jewish, though they were not religious. Her grandparents immigrated to Canada from Poland; her paternal grandparents were from Ukraine. In an interview in URB magazine, she recounted how growing up she experienced antisemitism; on her walks home from school, students from a nearby Catholic school would throw stones at her and call her a "dirty Jew".
Peaches is bisexual.