Seitz was a law clerk for Judge Charles Robert Richey of the United States District Court for the District of Columbia from 1973 to 1974.
She was in private practice in Florida from 1974 to 1996, and was a part-time adjunct professor at the University of Miami from 1984 to 1988.
Seitz served as president of the Florida Bar from 1993 to 1994; she was the first woman to hold that position.
President Bill Clinton nominated Seitz to the United States District Court for the Southern District of Florida on May 22, 1998, to the seat vacated by Stanley Marcus.
She was confirmed by the Senate on September 28, 1998, and she received her commission on October 1, 1998.