Bullen left Bayer, where she had been head of public and government affairs since 2017, to take up the role at MSD.
She replaces Jessica Fine, who had been in the role since 2016 and has moved to MSD's global comms team in the US.
Bullen reports to David Peacock, managing director, UK and Ireland, and leads a 22-strong external affairs team.
A public affairs specialist with more than 20 years' experience spanning high-profile agency and in-house roles, Bullen is responsible for MSD's UK strategic communications, policy and government relations.
She has a long track record in policy and comms for life-sciences and pharmaceutical organisations, and her main priority at MSD UK will be to promote the company and protect its reputation.
Bullen is also expected to ensure the external environment sustains future innovation, enhancing MSD's R&D and manufacturing presence and bringing its strategy and vision to life.
MSD is one of the biggest pharma companies in the world and employs more than 2,000 people in the UK alone.
The company said in a statement: "As well as bringing public affairs and communications expertise, Jo is an established leader with a wealth of people-management and organisational leadership experience."
Bullen added: "The external affairs department at MSD is renowned for being a talented and dedicated team and I feel incredibly privileged to have been appointed to this position. I am very much looking forward to getting to know the team and working with them."
Bullen spent eight years at Citigate Public Affairs, where she was an account director, before a nine-year stint at APCO Worldwide, where she was director of healthcare in its London office and joint head of its EMEA healthcare practice.
She went on to spend four years at Lexington Communications, as its international healthcare director, where she worked closely with Fine, who was head of UK healthcare. She stepped into Fine's job when her former colleague left to become external affairs director at MSD.
Bullen's move to MSD comes two years after she left Lexington for her role at Bayer.
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