The agency began the work last month. It is being led by the firm's London corporate team.
The contract is worth in excess of £250,000 ($330,000) annually and is set to run for several years, PRWeek understands.
Other agencies believed to have been considered for the brief include Edelman, FleishmanHillard, Ketchum and Portland.
Johnson & Johnson also works with Porter Novelli on a separate corporate account in the US.
Johnson & Johnson declined to comment, as did the agency itself.
Separately, Johnson & Johnson has been fighting a substantial reputational challenge in the US – it was ordered to pay out $417m in August as part of a lawsuit alleging a link between talcum powder in its products and ovarian cancer. US media reported this week that appeals by the company against that payment, and another related award, have been successful.
Its reputational defence has included maintaining the website FactsAboutTalc.com.
In recent months, Johnson & Johnson has also been in trouble for breaching the UK drug regulation code over promotions for smoking cessation product Nicorette, and lost senior US-based comms figure Amy Jo Meyer.