Mendelsohn will relinquish her executive duties at Karmarama in July to move to the social network and lead Facebook acroos the EMEA region. She will report to Carolyn Everson, vice president of global marketing solutions.
Mendelsohn replaces Shields, who left Facebook in January to head up the Tech City Investment Organisation, the government's scheme to create a Silicon Valley of technology businesses in East London.
Carolyn Everson, vice president of global marketing solutions for Facebook, said: "[Nicola] brings outstanding leadership and passion for what Facebook can do to become an indispensable partner for our clients and agencies throughout the region. It's testament to Facebook's innovative role in business and advertising that we're able to welcome a leader with such great experience."
Mendelsohn has held her role at Karmarama for the past five years and was the president of the Institute of Practitioners in Advertising for the past two. She is currently chair of the Creative Industries Council.
Under her tenure, Karmarama became the fastest-growing independent agency group in the UK working with clients including B&Q, Honda and Costa.
Mendelsohn will remain as a shareholder in Karmarama. Her management responsibilities will be divided between Ben Bilboul, group chief executive, and Charles Watson, group chairman.
Her marketing and business development responsibilities will be taken on by Hannah Matthews, the group marketing director.
Mendelsohn said: "My time at Karmarama has been unforgettable and it will always have a special place in my heart. The agency is truly unique, with a magical culture and some of the best talent in the industry.
"I’m proud to have been part of its story so far and I know that the team at Karmarama will continue to take the agency to even greater heights."