Hadden joins the agency in October after a spell running his own human resources consultancy. He will be based in London, but will also be involved with the agency's other offices in New York, Singapore, Shanghai, Tokyo and Sao Paulo.
Hadden will be charged with developing a talent management process to ensure that BBH attracts the best talent in the industry from around the globe.
He will maintain the network's succession management programme, which ensures that senior roles are filled internally when they become vacant. He will also oversee BBH's training and development strategy.
Hadden was previously on the executive board of WCRS and co-chaired Havas' global leadership group.
He was one of the 27-strong management team involved in WCRS's £20 million buyout from Havas in 2004.
Simon Sherwood, the chief operating officer of BBH Worldwide, said: "Our culture is very important to us and we are proud of our record of succession management and length of service.
"Both of these areas are critical to the success of our business. Niall will enable us to make sure that we are doing the best for our people globally and attracting the right talent in all of our offices."