Business Link London is one of nine regional arms helping new businesses to start up and establish themselves.
Selected after a six-way competitive pitch, Mandate will provide both PR and public affairs support.
On the PR side, the agency will focus on regional and business media engagement, using case studies and commenting on economic issues in the news agenda.
On the public affairs side, Mandate will also promote the organisation's work to the Greater London Authority, London MPs and councils, while setting up speaker opportunities.
The eight-month cam-paign will be overseen by Mandate director Jonathan Lomax, who leads a four-strong team.