Harris began his career as a child actor and was discovered by playwright Mark Medoff at a drama camp in Las Cruces, New Mexico. Medoff later cast him in the 1988 drama film Clara's Heart, starring Whoopi Goldberg and based on the novel of the same name by Joseph Olshan. Clara's Heart earned Harris a Golden Globe nomination. The same year, he starred in Purple People Eater, a children's fantasy.
Harris's first film role as an adult was 1995's Animal Room, although he portrayed a teenager. His subsequent film work has included supporting roles in The Next Best Thing, Undercover Brother, and Starship Troopers.
In 2004, Harris played a fictionalized, hyper-womanising, lewd version of himself in the Harold and Kumar stoner comedy films Harold & Kumar Go to White Castle. It was the shift from roles associated with his sanitized 'Doogie Howser' character to the more unscrupulous and bawdy part in the Harold & Kumar franchise that liberated Harris and revitalized his acting career, leading to his now-iconic role of Barney Stinson in How I Met Your Mother the following year.
