Harrison joins from Karmarama, where he was the managing director of retail, looking after B&Q, Costa and Iceland.
He previously worked at Mother between 2008 and 2012 as the global director of retail, heading up the Boots business. During that time the agency produced the "hear come the girls" campaign for the brand.
In June 2015, Campaign reported that Walgreens Boots Alliance was looking at its advertising agency relationships, at a global corporate level.
Tate said: "Mark is one of the leading experts in retail and the energy and ambition he brings for the brand is a great match for the Mother and Boots ambitions leading into 2017."
Harrison added: " I am delighted to rejoin Mother at such an exciting time, the forthcoming plans for Boots in the UK and on a global basis is groundbreaking and it’s amazing to be part of the team."