In 2366 he served as Ambassador Sarek's chief of staff. Like others, it became clear to him that Sarek was losing emotional control over time due to Bendii Syndrome. He knew of the help that Sakkath was providing him covertly and did not inform Sarek. Like Perrin, he believed that Sarek could complete the upcoming Legaran Conference, one last task to end his career with dignity.
When it came time for the trip to the Legaran system, Mendrossen beamed aboard the USS Enterprise-D with Sakkath to attempt to isolate Sarek. He told Captain Jean-Luc Picard that he would attend no events before the conference in order to stay focused. Sarek, however, asked to be shown the conference area anyway, and would not be deterred by Mendrossen. When Picard had found evidence linking Sarek to the random acts of violence on the ship, Mendrossen all but threatened him not to attempt to stall the conference, declaring Sarek was in fine health. However, when Picard confronted Sarek himself, Mendrossen was forced to allow Sakkath to explain that he had been helping to conceal Sarek's condition.