Garfinkel, who brings more than 23 years of engagement and change expertise, will be charged with delivering a broader range of specialisms to MSL and Publicis Groupe UK clients.
He has worked in-house and on agency teams for many FTSE 100 companies, and joins from FTI, where he was a senior director of its People & Change team.
Prior to that, Garfinkel headed internal communications at BAA/Heathrow Airport, TUI UK & Ireland, and William Hill.
At MSL, he will report to UK managing director Jo Grierson, who joined the agency from Talker Tailor Trouble Maker this month.
MSL UK chief executive Chris McCafferty said: "David's joining us at an exciting time as we push the business in new directions. Watch this space."
Garfinkel noted that MSL is one of PRWeek’s nine agencies to watch in 2020, adding: "I'm really eager to get stuck in at such an adventurous time."
In the past year, MSL's employee practice has added UK clients including Airbus and BT, which were won with Publicis' 'Power of One' approach.