McCormick was born and raised in Midland, Texas, and graduated first in her class from The Kinkaid School in Houston in 1977. She graduated with honors from Williams College in 1981 with a B.F.A. She also holds an M.F.A. from the American Conservatory Theater in San Francisco. She has worked in television, movies, theatre, and voice acting.

Her breakthrough role was in Enemy Mine, directed by Wolfgang Petersen with Dennis Quaid. Her other film credits include Woody Allen's Whatever Works, You Know My Name with Sam Elliott, and A Simple Twist of Fate with Steve Martin. She played Hannah's Mom in Barney's Night Before Christmas. Her first notable television credit was as district attorney Rita Fiori in Spenser: For Hire, in 1986–1987. She appeared as the holodeck simulation Minuet in "11001001", a first-season episode of Star Trek: The Next Generation, and later as Minuet (William Riker's holodeck love interest) in a fantasy-alternate universe during the fourth-season episode "Future Imperfect".

The role for which McCormick would become best known was as Dr. Elizabeth Olivet, a consulting psychologist for the prosecution on Law & Order. She appeared in approximately half of the episodes of the NBC series between 1994 and 2006.

Read more on Wikipedia