Trisha was born to Krishnan and Uma in Chennai (then known as Madras) into a Tamil Iyer family, whose ancestors hailed from the Palakkad region of Kerala She completed schooling at the Sacred Heart Matriculation School in Church Park, Chennai, and later pursued a Bachelor of Business Administration (BBA) course at the Ethiraj College for Women (Chennai). She ventured into modeling and appeared in several print and television commercials. In 1999, she won the "Miss Salem" beauty pageant, and later the same year, the Miss Madras contest. She had also won the Miss India 2001 pageant's "Beautiful Smile" award.
Trisha had aspired to become a criminal psychologist initially, and resisted the thought of pursuing acting, as she wanted to complete her studies first. She also appeared in Falguni Pathak's music video "Meri Chunar Udd Udd Jaye" as Ayesha Takia's friend. She was later approached for an acting role in Tamil movie Lesa Lesa by Indian film director Priyadarshan, which marked the beginning of her professional career as an actress. During her college course, she had occupied herself with a tight shooting schedule, which made it difficult to continue with her education. However, she compensated by attending summer classes.