McMahon's last episode as Supervisory Special Agent Jess LaCroix will air on March 8.
McDermott, who is currently recurring as Richard Wheatley on Law & Order: Organized Crime, will play a new character on the CBS crime procedural. His debut is slated for episode 17, which will premiere in April.
The casting comes just days after McMahon confirmed he was leaving after three seasons as Supervisory Special Agent Jess LaCroix on the FBI spinoff focused on the Fugitive Task Force. McMahon’s last episode will air on March 8.
“Over the past few months, the producers of FBI: Most Wanted and I have had discussions about my departure from the show in favor of additional creative pursuits and the transition of my character Jess LaCroix. These ongoing conversations have given us an opportunity to orchestrate a seamless and productive way for me to leave the show,” McMahon said in a statement to ET.
He added, “I would like to express the gratitude and admiration I have had working with [executive producers] Dick Wolf and Peter Jankowski. I’m extremely proud of the work we have done together and put the development of this show, and my character Jess, at the top of my professional experience list. I wish the show, and its cast and crew, the greatest success in the future. I am grateful to have had the opportunity to play Jess; he is a good man.”
In a separate statement, Wolf said, “We are saddened to see Julian leave. His portrayal of agent Jess LaCroix has been a key factor in the success of FBI: Most Wanted. We will miss him and we wish him the best in his next chapter.”