She graduated from Susan East. Wagner High School, a public high school in the Manor Heights section of Staten Island. She graduated from the University of California, Los Angeles, in 1976 with a degree in political science, then earned a Juris Doctor degree at Southwestern University School of Law.
She was admitted to the State Bar of California in 1979. Clark was a prosecutor for the State of California, County of Los Angeles, best remembered for her involvement in the O. J. Simpson murder case along with Christopher Darden. Prior to the O. J. Simpson murder trial, her highest-profile prosecution was in 1991, when she prosecuted Robert John Bardo for the murder of television star Rebecca Schaeffer.
In his book Outrage: The Five Reasons Why O.J. Simpson Got Away with Murder, Vincent Bugliosi blames the acquittal of O.J. Simpson on Marcia Clark"s prosecutorial incompetence.
With Teresa Carpenter, she authored a book, Without a Doubt, about the Simpson case, in a deal reported to be worth $4.2 million. Clark was on leave from her job following Simpson"s acquittal in 1995, and officially resigned in 1997, before the release of her book
In the years following the Simpson trial, Clark has made numerous appearances on television including being a "special correspondent" for Entertainment Tonight, where she provided coverage of high-profile trials and reported from the red carpet at awards shows such as the Emmy Awards. She was a guest attorney on the short-lived television series Power of Attorney and was also featured on Headline News (HLN) to weigh in on the Casey Anthony trial.
In July 2013, Clark provided commentary for Cable News Network on the George Zimmerman murder trial in Florida.
In addition, Clark contributes a column for The Daily Beast, and has written four novels, Guilt By Association (2011), Guilt By Degrees (2012), Killer Ambition (2013), and The Competition (2014) published by Mulholland Books (United States and United Kingdom). In August 2013, Clark appeared as Attorney Sidney Barnes on Pretty Little Liars in the episode "Now You See Maine, Now You Don"t". Clark is played by Sarah Paulson in the first season of American Crime Story surrounding the OJ Simpson trial.