Flynn started in the role last Monday (1 February).
He has more than 15 years of experience in corporate comms, both in-house and agency.
In June last year, Flynn left Eulogy, where he was head of corporate and professional services at Eulogy, where he created and developed a new division, working with clients including Tata Consultancy Services, Marks & Clerk and Santander.
Since leaving Eulogy, he has worked as a freelance consultant.
Prior in-house roles include leading the PR teams at law firms Pinsent Masons and Bircham Dyson Bell.
He said: "This is a fantastic opportunity for me to help develop and grow the team and ensure we continue to deliver market-leading work for our clients."
Sarah Scholefield, managing director UK & Ireland at Grayling, added: "In an increasingly competitive market, it was important for us to recruit someone who had the vision and skills to take our strong corporate offer, and to develop it further to becoming exceptional. David’s experience was exactly what we needed, and I look forward to working with him to continue to create measurable advantage for our clients."