The deal, which completed yesterday, will mean Walker Media will join Publicis Groupe’s ZenithOptimedia Group and form the "cornerstone" of a new global media network to launch next year.
The deal values Walker Media at £48 million. The business had gross assets of £49.1 million on 31 December 2012 and made operating profit of £5.2 million in 2012.
M&C Saatchi will retain 24.9 per cent and hopes its worldwide clients will benefit from the new global media network.
M&C Saatchi will be able to appoint a minority of directors to the board. Initially that will comprise David Kershaw, the chief executive of M&C Saatchi PLC, and Jamie Hewitt, the group finance director of M&C Saatchi PLC.
Kershaw said: "This new network will be of great value to our worldwide client base. Walker Media will benefit from the geographic reach and scale opportunities within ZenithOptimedia and Publicis, whilst importantly retaining the integrity of the Walker Media brand."
Campaign revealed the deal was set to go through last Thursday 21 November. It was expected to go through earlier this week, but was held up by contractual delays.