In the past few seasons, he has had a change in roles, leaving his permanent position as the lead medical examiner.
He is now the NCIS lead historian, which means he can come in and out of the story in each season.
Since season 15, he has been on the show in a limited capacity.
He tweeted about this back in April 2019 where he gave viewers an update on the situation.
McCallum posted an image of his cat and wrote: “I am delighted to say that I have reached an agreement with CBS for Dr Donald Mallard to stay with NCIS for season 16.”
He continued: “Once again it will be a limited schedule.
“That arrangement will allow me to spend time with Katherine, our children, six grandsons and Nickie, pictured below!”
NCIS season 18 is airing on Tuesdays on CBS.