As of May 2015, there is no indication that Mark Harmon plans to leave "NCIS." He signed a contract extension of unknown length in 2013, and CBS plans to air the show for at least two more seasons.
The details of Harmon's contract are private, but other show stars, including Michael Weatherly, signed two-year deals to continue in their roles. The show continues to enjoy excellent ratings and profitability, and Harmon is unlikely to relinquish his role as leading man and face of the show. CBS has renewed "NCIS" for the 2015-2016 season, which will be its 13th season on the air.