The London office will focus on attracting new subscribers in the UK, the US, Central Europe, the Middle East and Africa.
Six agencies from London and New York pitched for the business in a process that began in February.
There was no incumbent on the account.
Mark Cripps, the global head of marketing innovation and digital acquisition at The Economist, said: "[UM’s] data-led, programmatic and always-on approach to social media is exactly what we needed to manage this multi-market task.
"Their tier-one relationship with Facebook also unlocks alpha and beta tests that others couldn’t match."