Elder starts at the end of September as an associate director, reporting to MD Chris Norton.
Norton said: 'Elder will work across our digital and strategic consulting teams. He will bring his knowledge of digital, media and political campaigning to a range of our clients.'
Elder will join Blue Rubicon from MPG Media Contacts, where he is currently senior strategist.
He joined the Conservative Party's newly established new media team as web editor in September 2006.
As part of the Tories' widely praised digital operation alongside Rishi Saha and Sam Coates, Elder was instrumental in revolutionising the party's online presence up to last year's general election.
He helped to launch Webcameron, David Cameron's online video diary, which won the Hansard/Channel 4 Innovation Award and the New Statesman New Media Award, as well as an honorary Webby.
In the run-up to the election, Elder also led the party's efforts in developing the largest Facebook and Twitter presences of any UK political party and co-ordinating crowd-sourced rebuttals to government policy.
Earlier this year Saha, Downing Street's head of digital comms, joined Hill & Knowlton to become regional director across Australia, the Middle East, Africa and South and Central Asia, relocating to Dubai for his new role.