She joins the management team at McCann, working alongside the chief executive, Stephen Whyte, the managing director, Chris Macdonald, and the executive creative directors, Brian Fraser and Simon Learman.
Crumpton joined Fallon at the agency's launch in 1998. Most recently, she headed the planning department in the role of planning partner. She joined the agency from BMP DDB.
Whyte said: "We are thrilled to have secured Nikki from Fallon. She is undoubtedly one of the most talented thinkers in the industry."