The specialist trust, which provides mental health, learning disabilities, addictions, brain injury and dental services, selected the Southampton office of the UK-wide agency after a competitive pitch process.
The agency will promote the trust's services to local media, along with developing ideas for a national programme targeting specialist publications such as Health Service Journal.
Part of the agency's brief is to encourage more local people to become members of the trust since it was awarded foundation status last year.
Trimedia senior consultant Paula Quigley said: 'A big part of our work will be breaking down the stigma of mental health services. There are a lot of misconceptions about mental health.'
Quigley is leading the account in conjunction with executive director Caroline Searle. The agency won the account after a three-way pitch.