Cahill was born in Stafford, Virginia. She began acting in her mother's local high school productions at age four, and started singing and dancing lessons at age eight. She was Miss Pre-Teen Virginia in 1991 and the first runner-up for Miss Junior America. She continued to act into her high school years at Brooke Point High School, where she served as the president of many clubs, including the Honor Thespians group. As a junior, Cahill was the only one of her year to garner perfect scores in all phases of her audition to the Governor's School for the Arts in drama. At age 16, she also won Overall Actor in the Actors, Models, Talent for Christ competition. She was further selected into the Barton and Williams Dance Company, winning both company and individual awards. Following high school graduation in 1998, she attended Marymount Manhattan College in New York on an academic and performing arts scholarship. She left college at age 19 to pursue a full-time acting career in Los Angeles.