The FTSE 100 insurance giant put the account out to pitch in October, with Lexington invited to repitch.
The final decision is a blow for Lexington, which had been fighting hard to hang on to one of its biggest accounts.
Open Road now has a team of 12, having been set up in 2007 by former Fishburn Hedges public affairs chief Graham McMillan, along with three others.
The pitch process was overseen by Prudential group public affairs director Miles Celic, a former Fishburn Hedges consultant.
Open Road lands lobbying account with insurance giant Prudential
Prudential has dropped Lexington as its retained public affairs adviser, turning instead to rival firm Open Road.
