The 69-year-old will recur in four episodes as Marcie Warren, who is a "a seasoned investigative journalist who uses her savvy wit and down-to-earth people skills to aid in her tireless pursuit of truth," starting on April 6.
Dawber is best known for starring alongside Robin Williams in Mork & Mindy, as well as for her work in My Sister Sam.
The couple tied the knot in 1987, but this is the first time they have ever acted together in the same project. "Their secret is they put their family first above all else, including work," a source previously told OK! of Harmon and Dawber’s strong marriage.
"Mark prefers to stay out of the spotlight," the insider explained, "and while Pam still takes the odd acting job here and there, she loves tending the garden, cooking and making a beautiful home for her family.
"Mark’s changed over the years. He’s mellowed with age. Mark spends afternoons doing carpentry, working on his cars or cooking with Pam. Those are the moments he cherishes."
Fans can also expect to see more new characters as season 18 continues. Hawaii Five-O alum Katrina Law will be stepping in for the last two episodes as Special Agent Jessica Knight, who is a hostage negotiation specialist. Law may continue in the show after season 18 but nothing has been confirmed yet.
Meanwhile, Maria Bello stepped down as Jack Sloane this season when she told Harmon's character that she wanted to leave the NCIS and relax. Bello did not renew her contract after she signed onto NCIS in 2017.
However, it is unclear if Harmon will stay with the show. Harmon has played Leroy Jethro Gibbs since the show premiered in 2003, but this may be his last season as a series regular with his contract expiring this year. In the Tuesday, March 16, episode, Gibbs handed in his badge and gun, which raised questions about his future in the NCIS.
However, he will reportedly star in a "handful" of episodes in a 19th season amid reports that CBS would allegedly cancel the show if he left.
"The responsibilities of the show seem to wear heavier on Mark by the year," a source previously told OK! as Harmon is also an executive producer of the drama. "Even on a break, you can't get him to crack a smile or tell a joke. He'd rather go lie down in his trailer."
Meanwhile, NCIS: New Orleans has been canceled, but fans have the new spin-off NCIS: Hawaii to look forward to.
Entertainment Weekly was the first to report on Dawber’s role on NCIS.