Merkerson asked to be released from her contract on the long-running series, says The Hollywood Reporter.
The 57-year-old has played Lt Anita Van Buren on the show for 19 years. She's been on the programme longer than any other cast member.
The actress won an Emmy Award in 2005 and a Golden Globe in 2006 for her performance in HBO's Lackawanna Blues.
She has also appeared in the films Black Snake Moan and Mother and Child.
Law & Order is currently in its 20th season. It is unclear if NBC will renew the police drama for another season.