PRWeek reported in October that the magazine was speaking to agencies following its sale by Pearson.
Speed Communications has managed PR for The Economist for the past ten years.
The pitch process was run by Holly Donahue, senior manager, brand comms.
Charles Barber, vice-president, PR and thought leadership at The Economist, said: "Following a robust tender process, which included our editorial colleagues, we have appointed Lansons with immediate effect to work with us. The Economist’s paid-for circulation is growing and we would like to ensure we are seen as a must read for all progressives including the next generation. The Economist’s innovation is already widely recognised with new apps like Espresso and Lansons will help us as we continue to push for change and drive innovation."
Louise Ahuja, director at Lansons, will be leading the account. She added: "I feel excited to be working with such an iconic brand in British life. The Economist editorial team is a leading force in journalism as well as driving innovation in how news is consumed."
In August last year The Economist Group, which publishes the magazine, announced that Pearson's 50 per cent stake in the group would be acquired by investment company Exor, and the group itself.
The Economist currently has a total circulation of 1.6 million while its digital circulation grew by 226 per cent year-on-year. The Economist Group also owns The Economist Intelligence Unit, CQ Roll Call and TVC, the PR and digital marketing agency.