McCann joins from Tag where she held the post of director. Prior to that, she was managing partner at MEC.
In her new role, McCann will be responsible for client relationships, leading digital dialogue and "bringing together the agencies talent to connect paid, owned and earned to deliver on the clients business objectives", according to Havas Media.
She takes up the post following the promotion of Natasha Murray to managing director in July during the company’s senior management restructure, which also included the promotion of Paul Frampton to chief executive.
Marc Mendoza, the Havas Media chairman and former chief executive, decided to leave his position in September.
McCann said: "I was drawn to Havas Media because of its dynamic client-centric model and the unique combination of digital capabilities and specialisms to deliver innovative integrated solutions. I feel excited about the opportunity of taking the agency’s client leadership to the next level."
Cox has been promoted from head of strategy to replace Hamid Habib, who left the agency earlier this year for OMD UK. Cox joined Havas Media in 2002 as strategy director on its National Lottery and Orange accounts. She has since worked on client business including AXA, P&O Ferries and EDF Energy.
She will work across all Havas Media strategy disciplines, from communications planning, idea facilitation, futures and innovation, digital strategy, data planning and analytics and effectiveness.
Cox said: "[My new role] will allow me to do what I enjoy most – get under the skin of how to best integrate traditional and new communication channels, ideas and opportunities and work out ways of understanding their effectiveness."
Murray welcomed the pair to their new roles and praised their talents to "apply strategic thinking to providing solutions will be indispensable in taking the agency to the next level".