McKinnon an haife shi kuma ya girma a garin Long Island na Sea Cliff, New York zuwa Laura Campbell, mai koyar da iyaye, da Michael Thomas Berthold, masanin gine-gine. Tana da 'yar'uwarta, Emily Lynne. Mahaifinta ya mutu lokacin da take da shekaru 18.Yayinda yake yaro, McKinnon ya buga kayan kida da yawa. Ta fara kunna piano lokacin da take da shekaru 5, da cello lokacin da take 12, kuma ta koya wa kanta yadda za ta yi wasa da guitar lokacin da take da shekaru 15. Ta kammala karatu daga makarantar sakandaren North Shore a shekara ta 2002, kuma daga Jami'ar Columbia a shekara ta 2006 tare da digiri a wasan kwaikwayo, inda ta kafa ƙungiyar wasan kwaikwayo, Tea Party, wanda ya mai da hankali kan wasan kwaikwayo na kiɗa. A Columbia, ta fito a cikin wasan kwaikwayo uku na Varsity: V109 "Dial for Deadline", V110 "Off-Broadway" da V111 "The Sound of Pranges".
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