McKinnon mụrụ ma zụlite na Long Island obodo nke Oké Osimiri Cliff, New York ka Laura Campbell, nne na nna onye nkụzi, na Michael Thomas Berthold, onye na-atụpụta ụkpụrụ ụlọ. O nwere nwanne nwanyị, Emily Lynne. Nna ya nwụrụ mgbe ọ dị afọ 18. Dị ka nwatakịrị, McKinnon na-egwuri egwu ọtụtụ ngwá. Ọ malitere igwu piano mgbe ọ dị afọ 5, cello mgbe ọ dị afọ 12, ma kụziere onwe ya otú e si akpọ guitar mgbe ọ dị afọ 15. Ọ gụsịrị akwụkwọ na North Shore High School na 2002, na Mahadum Columbia na 2006 na ogo na ihe nkiri, ebe o jikọtara otu ìgwè ndị ọchị, Tea Party, nke lekwasịrị anya na ntụrụndụ egwu. Na Columbia, ọ na-eme ihe ngosi atọ nke Varsity: V109 "Dial for Deadline", V110 "Off-Broadway" na V111 "The Sound of Pranges".
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