In 1970, Principal moved to Hollywood. She won her first film role as Marie Elena, a Mexican mistress, in John Huston's The Life and Times of Judge Roy Bean (1972) (opposite Paul Newman). She got a Golden Globe nomination as Most Promising Newcomer for this role.
She starred in many movies and TV programs. Her most notable acting was playing Pamela Barnes Ewing in the nighttime television series Dallas. She spent nine years on the long-running series, leaving in 1987.