In July 2008, Prescott and twin sister Megan appeared in an episode of soap opera Doctors, playing twin sisters Amy and Charlotte Wilcox in "Dare, Double Dare, Truth". Beginning in early 2009, Prescott played Emily Fitch in series 3, 4 and 7 of Skins, with characters known as the "second generation". Prescott stated on her official website in March 2010 that she had a role in the pilot of Goth.
Kathryn is six minutes older than her twin sister Megan. In providing her "Top Ten Playlist" on the Skins official website, Prescott named Röyksopp, The Cardigans and Metallica as her favourite music. She and her twin sister Megan went to St. John's Senior School for secondary education before they went to separate schools thereafter. In response to a fan's question on her official website, Prescott revealed that she would prefer to not label herself regarding her sexuality, and that 'labels are for cans' (following rumours regarding the homosexuality of her character in Skins), but does not believe that "people are defined by their sexuality", suggesting that "it doesn't change who you are as a person". Following another question, she revealed that while she is not 'strictly' a vegetarian, she is wary of what meat she consumes.