Buchanan has been chief executive of Publicis London for the past six years. She will now take on the new role of chairman of the agency as well as chief client development officer at the wider Publicis UK group.
Buchanan will oversee client relationships across Publicis UK’s five agencies: Arc, August, Chemistry, Poke and Publicis London.
Farnhill, co-founder of Poke, remains chief executive of the digital shop. In his new role, he will focus on growing digital communications and experiences for clients.
Poke will remain a separate agency, with co-founder Nik Roope as its executive creative director.
Buchanan and Farnhill continue to report to Guy Wieynk, chief executive of Publicis UK and western Europe, and global lead of Sapient Inside.
Wieynk said: "The new structure will provide clients with easier access to digital communications, digital
experiences and Sapient Inside’s enterprise technology offering."