The agency was appointed to the six-month remit after a three-way pitch.
It will handle media relations on the project, which aims to de-stigmatise mental health.
Mental health was the focus of a draft Mental Health Bill, published this week, which would give doctors powers to force patients to receive treatments including drugs and therapy.
Pfizer is spearheading the 'Art Works in Mental Health' campaign, and is in partnership with NHS Trusts and charities including breast-cancer body Breakthrough.
Harvard Health's account team - led by managing director Paul Jarman - will centre its campaign on the role of partnerships, from drug companies through to charities, within the mental health service in the UK.
Jarman said: 'Our campaign is aimed at the de-stigmatisation of mental health. The arts exhibition will provide the focal point and will showcase the talent of artists to a massive audience.
'We aim to use the visually-striking content to challenge perceptions of the contribution people with mental health problems are capable of making in commerce and society,' added Jarman, who reports to Pfizer mental health programme chief Michael Warren.
The arts exhibition will travel the country from 2 July, calling in five cities.
Harvard Health will be responsible for boosting regional press interest in the programme, which will exhibit art and creative writing by people with mental illness.
Harvard Health is part of Chime and merged with Radius Healthcare (PRWeek, 8 March) almost four months ago under the combined HarvardHealth brand.
In April, Pfizer launched two major comms campaigns involving football legend Pele and Australian sex therapist Dr Rosie King to promote its flagship anti-impotency drug, Viagra (PRWeek, 5 April).