Davies had spent the last five years at WE Communications, initially as EMEA digital lead before adding the role of UK consumer lead to this title last summer.
In his new role, he will report jointly to UK MD Paddy Herridge and New York-based chief digital strategist Parker Ray.
Davies had set up MSL London's digital practice in a five-year stint earlier in his career.
He will oversee digital strategy for all UK and EU clients in his new role in line with the agency's "focus on delivering fully integrated and earned-worthy programs", it said in a release.
MWW currently has 25 staff in London out of a total of circa 300 globally. Major UK clients include News UK and Sky Media, while recent wins include fintech firm Global Payments and telco and tech provider CenturyLink.
The agency's founder and CEO Michael Kempner said: "Building our UK digital team to complement our award-winning US resource will enable us to offer an even greater breadth of integrated campaigns and digital services to UK and EU clients. Gareth's experience and insight-led approach will help maximise how we tell our clients’ stories across all platforms and grow their businesses."