May is a former director at RELX, Centrica and SABMiller, and brings more than 30 years of experience in communications and management. She will help the recruitment firm "deliver new and improved services to the communications, corporate affairs and advocacy functions".
Aside from recent board duties, May is a leadership coach and works with senior leaders in communications and other sectors to help them "perform and exceed their goals".
Additionally, she sits on the boards of The English National Opera, The Pensions and Lifetime Savings Association and South Staffs Water.
Ellwood Atfield managing director Ben Atfield added: "I believe Catherine’s insight will be an asset to us as we develop our coaching and mentoring hub for corporate affairs leaders, and new talent management and organisational design consultancy services."
Ager will join the company in its Brussels office. He has served as the secretary general of the European Round Table of Industrialists, a forum bringing together CEOs and chairmen of major European multinationals from 2011 until 1 February 2019.
He was previously the director general of the European Federation of Pharmaceutical Industries and Associations (EFPIA) from 1995 to 2011.
Ellwood Atfield Brussels managing director Mark Dober said: "We are delighted that Brian is joining our senior Brussels team which already includes four former business leaders. I have had the pleasure of knowing and working with Brian for 15 years, and am certain his insights, intellect and network will add real value to our work."