Trinick will join M&C Saatchi later this month, working on the £60m creative brief it won in May last year, which includes community engagement as well as creating videos, websites and posters.
A spokesman for the company clarified that the client for that brief - the Home Office's Office for Counter Extremism - is "completely distinct" to the Office for Security and Counter Terrorism.
The latter runs the controversial Prevent programme, which does not involve M&C Saatchi, although it is still part of the Home Office. "We make this distinction because the two are often unhelpfully confused," the spokesman said.
Trinick's final day at PLMR is 12 January.
She joined what was then a 10-person agency in 2010 as an account executive, and while the agency headcount has quadrupled, progressed to director level, joining the board in December 2015.
PLMR CEO Kevin Craig said Trinick had "always been tireless in her commitment to deliver the very best service to her clients, and to grow PLMR into one of the UK’s leading agencies", and that he and the agency would miss her work.
He went on to say: "We are grateful for all her hard work, and wish her the best of luck in her new role doing work with a great agency on a cause crucial to our country. She will remain a dear friend of the business."