The 70 employee-strong agency, which will be renamed Salterbaxter MSLGroup, will retain its management team, with Nigel Salter as CEO and Penny Baxter as MD.
Salter will also take on the role of EMEA head for MSL’s corporate and brand citizenship practice, working with global head of the practice Scott Beaudoin.
Of the acquisition, MSLGroup EMEA president Anders Kempe said: "The issues of sustainability, transparency, trust, accountability, social purpose and long-term value creation increasingly shape how business intersects with society.
"As clients look for trusted advisers to help them develop effective strategies and tell compelling stories against this backdrop, we’re delighted to have Salterbaxter join the growing MSLGroup team in London, and our dedicated practice operating worldwide."
Salterbaxter was set up in 1998 and has since worked with clients including Adidas, Anglo American, BP, BSkyB, Lego, Maersk and Thomson Reuters on business strategy and sustainability.
It was guided on the deal by corporate advisory firm Clarity.
The acquisition means that together with Salterbaxter, MSL's corporate and brand citizenship practice will now number 170 staff.
Salter added: "The shifting global agenda has offered us an opportunity to scale and accelerate our business. MSLGroup’s important geographic and client footprint; its expertise in corporate affairs, digital and social, consumer communications, employee engagement and reputation management, and the company’s progressive approach, will help us achieve that strategic expansion."