Hancock, previously the planning director on the agency's Royal Mail account, replaces Simon Calvert, who left to join FCB last year.
In his new role, one of his first tasks will be to strengthen Proximity's senior planning team by making internal promotions and appointments. He has already promoted Kevin Allen to the position of head of planning.
Hancock joined Proximity London in 2004 after a two-year stint as a freelance.
During this time, he worked for JWT, OgilvyOne and RMG Connect. Before that, he ran the communications agency Red Square, which he founded in 1993. He began his advertising career in 1991 as a freelance art director.
Amanda Phillips, the managing director of Proximity London, said: "Proximity London has a proud heritage of delivering world-class planning capability and Mark was the obvious choice to continue this legacy in the role of strategy director.
"Over the past 15 years, Mark has worked with clients across a broad range of sectors and proved his ability by delivering campaigns for many of our leading clients. In Mark and Kevin, we have a senior planning team that will keep Proximity London at the forefront of the industry and deliver the superior strategic counsel and planning our clients have come to expect."
Hancock added: "I am looking forward to delivering a clear vision and strategy that will enable us to create breakthrough, behaviour-changing ideas for our clients."