The agency won the account, thought to be worth around £200,000 a year, via a competitive six-way, two-stage pitch.
It replaces Weber Shandwick, which had previously held the account since 2007 and declined to repitch.
Lexis will look to help the firm consolidate its existing customer base and draw in new audiences, particularly targeting young adults.
Consumer board director, Liz Bendall, will head up the retainer account, reporting to EMEA head of comms, Matthew Knott.
RBP International, which handles Harley-Davidson’s PR in the UK and Ireland, renewed its account for the fifth consecutive year last month, running to the end of 2011.
When Weber won the account in 2007, its primary focus was working with local markets to ensure the Harley-Davidson brand was seen as both relevant and attainable.
Weber declined to comment on why it did not repitch for the brief.