McCullah was born in the Philippines, where her father had been assigned with the United States Navy performing counterespionage duties. She grew up in Bergen County, New Jersey, and graduated from Indian Hills High School in her hometown of Oakland, New Jersey. As a high school student, she maintained in her diary 10 Things I Hate About Anthony, her boyfriend at the time, which ultimately led to the title of her 1999 film, 10 Things I Hate About You.
