Redleaf won the account after pitching against Weber Shandwick, Pelham PR and the incumbent.
Other agencies submitted initial proposals, but did not make it to the final round.
The group's last trading update, at the end of March, was handled by Smithfield and saw the firm announce it was moving in line with expectations.
For the full year to 31 March (figures to be published on 21 May), the company expects revenue to have grown by more than 6 per cent on last year's figures to at least £565m.
Redleaf will be able to promote some notable recent wins for the company, including a brief to provide corporate voice and mobile services for South Somerset District Council.
The company will provide a system that will carry voice traffic over its 'calls and lines' service for more than 50 locations across the council's area, as well as provide mobile services to 300 employees.
Smithfield, which had held the Thus account for about seven years, is currently the subject of industry rumour of an imminent sale. The agency has denied that it is in talks with part-owner Chime.
Huntsworth-owned Hudson Sandler has also denied that it is about to purchase the well-respected agency.