The two acquired sister businesses are Red Hare Digital, a talent agency which manages beauty influencers, and Grey Whippet, an influencer marketing consultancy.
Terms of the deal were not disclosed.
They were founded by Jonathan Poole and Kate Lovett and together employed close to 20 staff.
Poole and Lovett will lead the newly created M&C Saatchi Social, an influencer marketing and talent management agency. It encompasses the acquired businesses and a further 20 existing group staff. The pair, and other staff, will remain minority shareholders in the business.
M&C Saatchi Social sits alongside M&C Saatchi Merlin, the group's talent management agency, as part of The M&C Saatchi Talent Group.
Richard Thompson, who has taken up the role of chair of The M&C Saatchi Talent Group, having previously chaired Merlin, said: "The world is always changing and M&C Saatchi’s agility and entrepreneurial strategy enables us to adapt and remain at the forefront of what our clients need.
"The marriage of brands, insight and the scale of the M&C Saatchi global network provides an offering that is totally unique. Through the acquisition of Red Hare and Grey Whippet, some of the most successful and exciting creators in the UK become part of the group."