McGillis attended the Juilliard School's Drama Division, where she performed in William Congreve's "Love for Love" directed by John Blatchley. She graduated in 1983 and began landing acting roles shortly thereafter. Her breakout role was that of an Amish mother in the movie Witness with Harrison Ford, for which she was nominated for a Golden Globe award. Her next high profile role was that of flight instructor Charlie in the 1986 fighter-pilot movie Top Gun with Tom Cruise and Val Kilmer. After 1988's The Accused, McGillis appeared in dozens of television and movie roles throughout the 1990s and then took a break from television and film for a few years. In 2004, she appeared in the stage play The Graduate as Mrs. Robinson, touring the United States. In 2006, she began working in television again, and then in 2007 she joined the cast of Showtime's The L Word for its fifth season.