The ongoing brief, which began this month, includes media relations, social activity, event support and strategic campaign support.
In particular, the agency - which is part of Cubo Group - says in a release it "will create and execute a series of strategic campaigns that showcase why a Brompton bike is perfect for both work and play in the modern city as it looks to reach a new fan base".
The Earned Agency was appointed after a four-way pitch.
Stephen Loftus, chief sales and marketing officer at Brompton, said the agency's "blend of expertise in traditional PR, social media, search and content marketing all underpinned by good strategic thinking really appealed to us", adding: "It demonstrated a thorough understanding of the brand and had some bold ideas as to how we can take our messages to a wider audience."
Neil Foster, managing director at The Earned Agency, said Brompton was "an appealing brand with so much depth and heritage", commenting: "We are really looking forward to leveraging our expertise among active lifestyle brands to help drive sales and let more and more people realise the convenience and freedom that owning a Brompton brings."
The Earned Agency had previously worked with Brompton on a project, launching an Instagram-based film competition #MyUnseenCity in November as part of a drive to reach a younger market.