The former Battlestar Galactica star, 58, will reprise her role as Capt. Sharon Raydor full time. She previously recurred during Season 5 and 6.
McDonnell's promotion puts the future of The Closer further into question. Despite star Kyra Sedgwick's impending departure from the show, TVLine reported in January that TNT was hoping to keep the hourlong drama alive, either with a new lead or via a spin-off.
The site now says the cable network is likely to spin off The Closer into a new show called Major Crimes, however there is no word yet as to whether McDonnell will be involved.
The new season of The Closer premieres Monday, July 11 at 9/8c on TNT.